I won't talk about the small-scale structure of buildings, since the answer by type_outcast has that well-covered. However, I will mention the feasibility issues in constructing buildings suitable for a variety of creatures. The height of the giants is the limiting factor. The minimum height of a ceiling should be such that if the largest person likely to enter a building raised their hands, they wouldn't touch the ceiling. Otherwise, the construction may well feel cramped. Of course, grander buildings are often significantly taller than this criterion would suggest. Storm giant adults are typically 21 feet tall. If their heights follow a roughly Gaussian distribution, as is the case with humans, and we assume that the standard deviation is simply scaled up with the mean, then storm giants who are 23 feet tall are feasible within a modestly sized population. Regardless, a Huge bipedal creature is assumed to be able to reach 32 feet without jumping.
So we see that a ceiling must be 32 feet high, merely to prevent storm giants from brushing the ceiling when they reach up and stretch. The best bet for constructing ceilings this high is stone. However, your technology is barely coming out of the Stone Age! You might have trouble making reliable ceilings 32 feet tall or higher with technology alone.
You're a wizard, prospective architect
Fortunately, you have magic. The best bet is the Moaning Diamond. Yes, this is a major artifact, but it is truly powerful. It can reshape 5,000 cubic feet of stone (or earth for that matter) three times per day. With a decent knowledge of engineering (or some trial and error), quite high stone ceilings are plausible.
Let's say that you make the walls 9 feet thick (like Notre Dame). Of course, this will only be at the base, and is certainly excessive, but it serves as an upper bound. Then a 555 square foot area can be constructed each day from the underlying stone, which in a mere ten days would allow the construction of a cubic stone building 52 feet on a side. Obviously, the building could be even larger, since the stone would be much thinner at the top, and 9 feet, even at the base, is very thick indeed (Notre Dame is about 100 feet tall under the vault, far larger than the minimum needed by our giants). But the point is, with such powerful magic, an area with a great deal of underlying stone, and the insane intelligence of a high-level wizard or such, the construction of giant-size buildings would be child's play.
There are other techniques, of course. Stone shape is a weaker alternative to the Moaning Diamond, which at level 20 can reshape 30 cubic feet per use. A cleric with the Earth domain can theoretically cast this spell 35 times per day, which comes to 1050 cubic feet of stone per day. This is about one-fifteenth the strength of the Moaning Diamond, but a building of the sort mentioned before could be crafted in about 150 days, which is not all that bad. Keep in mind, though, that this is a single building that a storm giant might find comfortable, and it requires a powerful cleric to lock up all their high-level spell slots for months on end building it. If the cleric wanted to be prepared for their enemies, or minister to the sick, the construction could take a year or more.
One should note storm giants around could vastly simplify any large projects, due to their height and prodigious strength. However, the basic architectural issues would still exist, and would likely better be solved by stone-shaping.
But wait! There's more....
There is even a construction method perhaps even more potent than the Moaning Diamond: polymorph any object. Suppose a 20th-level sorcerer or wizard transmutes a nonliving mineral object of up to 2000 cubic feet into something (a) the same size, (b) also a mineral, and (c) as a mineral, of the same Intelligence (0). This transmutation will then be permanent, allowing a caster to (for example) create iron reinforcements in a wall created with stone shape, or glass windows, or indeed metal doors. It is probably wise to require that only contiguous, firmly bound "objects" (which also rules out fluids of any sort) be affected by polymorph any object, so as to avoid players turning the ground beneath their enemies' feet to polonium or something. After all, transmute rock to mud is one thing--transmute rock to plasma is quite another. In this case, of course, the stone will need to be moved away from the portion to be transmuted, and the portion held in place by telekinesis or such while it is transmuted, and then the stone sculpted back in. Nonetheless, the potential for extremely sturdy buildings is there.
Building for pixies is barely harder than building for giants. Pixies may not have the strength to construct houses easily from the inside, and humans (let alone giants) would lack the ability to construct such fine structures. However, a sufficiently powerful wizard-architect could build a scale model. It would have to be less than 40 cubic feet, because of certain spell limitations. This is large enough to construct details like doors and such with stone shape, or simply to add them manually, while small enough that quite a lot of artistry (whether magical or mundane) could be applied. Then, boom! Permanent shrink item, and stick the pixie apartment into the pixie condominium. Stone shape them together. Rinse and repeat.
So the gist of this is that although Stone Age communities would have trouble building habitations large enough for giants, or small enough for pixies, a single powerful spellcaster, or anyone equipped with a powerful artifact, could manage it extremely quickly. Some time would be needed for trial and error, of course, since one does not learn architectural principles in a day--but being able to shape a new building from solid stone every few days would definitely help the learning curve.
Cat flaps for pixies (and humans)
Well, we know how to construct massive stone buildings for giants. How do we make sure that everyone can enter the building? Clearly the highest priority is to build massive doors for the giants. Now, we could put small doors in for the humans, and smaller doors in for the pixies. But come on! If you are a 20th-level wizard constructing a city with an artifact of the elemental lords, are you really going to bother making three doors when you could make two?
No. You make a permanent phase door. Any creature of size Medium or smaller can use one, as long as you put the right conditions on it! Bonus: no doorknobs, so even creatures without hands can enter. And, of course, cats.
Cat flaps are for lesser mortals! I shall make a single door!
It is true. Why make two doors when you can make one? In fact, why not make...no doors at all? Enter the permanent teleportation circle (actually, two). Think of it as a doormat that also acts as a door. For someone with the finances of a 20th-level character, plus those of a city, even the high gp cost of this spell will be manageable. The circle will be set to teleport anyone who activates it a mere few feet, into the building itself. This includes creatures of any size, including our Huge storm giants. Of course, there may need to be emergency doors, in case some fiend (literal or otherwise) disjoins your nice teleportation circle. However, barring such catastrophes, this solves the problem of having to create three doors, or force pixies to struggle with human doors, or humans to struggle with giant-sized doors.