You should also have included an introduction to the physics of your world. As in, the type of planet (if it is other than earth), it's atmospheric composition (especially oxygen level), the characteristics of other creatures in the world (are they all lean and large or only humans?) and anything else you have placed differently from earth.
In my answer I am assuming these lean characteristics are limited to humans of the world and other species are as heavy as their counterparts are on earth.
It would have helped if you also stated WHICH hair color was the norm for your people. Anyhow, for generally "light" colored hair, you can use diet preferences. For example, your human(oid) creatures have been eating cereals x, y and z for millenia. These cereals have large quantities of zinc/iron/copper salts in them, which gradually found their way in the hair coloring protein. Similarly if they are also a carnivorous peoples, you can add the same metal salts mention for their meat diets to explain their hair color. Only now those salts are also present in the meats of the animals they hunt and consume. You can also make an interesting grouping of your people by hair color too. People who eat more mutton than vegetables have light brown color hair. People who eat more vegetables have rather grayish hair. People whose diet mainly consists of fish ...
Here I am assuming that by "light hair", you are implying light-colored hair. If you intend to mean "less hair" or "thin hair", please state so clearly.
This can be a result of:
a) Sexual Selection. During the early stages of these human(oid) evolution, taller males were preferred by the females. Taller males having more access to females than shorter males would mean more babies born of taller males which would increase the chance that a taller baby would be born. Repeated over millenia, the general height of these people came out to be 7 feet which is the tallest height supported by their skeletal structure. People can grow taller than that, but they will have problems walking and daily life activities so they are not much preferred by the females. Hence the general height of the individuals is set at 7 feet.
Now comes the very important question of WHY the females preferred tall males? One answer can be that the region where early human(oid)s lived, was grassland area and the grass was really tall. A tall male was more likely to see above this green layer. His and his mate's life would be somewhat easier than a short male, hence females preferred taller ones.
Or/And you can specify that the trees in the area of early evolution stages had their lowest branches sprouting at quite some height. So in times of danger a tall male had better chances of jumping and grabbing a tree branch to save himself and later pull his mate on the tree too.
b) Survival Selection. As in, tall males were more likely to survive in the early stages of these human(oid)s and hence had more chances to reproduce. The difference here is that females don't have a mate preference based on height of males, but taller males are more likely to survive while shorter males died before reaching reproduction age.
Once again you are going to have to throw in reasons of how/why taller males were more likely to survive where shorter males perished.
Were taller males better suited for hunting? Foraging? Was their ability to see for longer distances a key factor?
This is more easier accounted for, than height. We can use two approaches here.
a) Diet. The plants/animals consumed by the human(oid)s contain very little phosphorus. This lack of phosphorus would imply slender, lightweight bones. Lightweight skeleton leads to a thinner muscles and a generally lean build.
You can also use a lack of calcium in their foods but I would rather not prefer that. Calcium is also a key constituent of teeth and you might not want to give them weaker teeth.
b) Survival/Sexual Preference. Lightweight individuals run faster than heavy ones and this can be the key difference between getting caught by a chasing predator and outrunning it till you reach a shelter. Or, for reasons lame and unknown (known only to women), the females could prefer skinnish males over heavier ones.
Longer, Prehensile Toes
This can be easily explained IF your peoples are in early stages of leaving the jungles and settling on the ground. Or maybe they still have to climb trees often for one reason or another. OR maybe they have this super ability to pick up objects with their feet as well as hands, which makes them far more successful engineers/craftsmen than others who can only use their hands for picking up objects.