Unless you really need the detail, let's simplify this a bit
In the end, you should simplify this a lot. You'll see why as we walk through my "simplification."
To begin, if you read this article about building NASA's Pad #39, which is a serious simplification for the public, you'll quickly realize that addressing the problem a pad with any realism is probably way, way, way beyond what you want.
Worse, there's a difference between landing with parachutes in the ocean and landing on wheels like the Shuttle. Or maybe you have ships that land like a Harrier jet or use some form of gravitics to gently land. Let's not get distracted. Your ships look like bigger versions of the Space Shuttle.
Worse still, then there's launch. And it's pretty rare that the launch requirements and landing requirements line up.
So, unless your ships can't land in the proverbial field like your average Cessna propeller aircraft can, then there is a minimum colony population for the size of pad needed and it won't be twelve people. The investment in launch and landing facilities is a big honking deal. (I'm a fan of reminding people that you really can't ignore economics when worldbuilding.)
So, you need:
A landing strip capable of dealing with the incoming velocity of the landing craft and its mass.
A launch pad capable of dealing with the weight of the entire launch assembly and the force of lift-off.
Simple... we need to keep this simple...
It doesn't make sense to haul all the materials to build these to a new planet when the new planet is sitting there with all the materials you need. Especially when the colonists will continue to benefit from the infrastructure you put into place to make this happen. So, what do you need?
- Gravel pit
- Cement factory
- Iron ore mine
- Smelting and refining
- Equipment... lots and lots of equipment and the ability to fuel it and maintain it.
- Transportation (see #5)
- People. Thousands if not tens of thousands of people.
Granted, the construction crew might leave to build the next planet's transportation hub, but if you think about it, that doesn't make sense. Your colony will need pretty much everyone involved.
If you want to worry about detail...
- Housing & Recreation
- Food, water, and sewer
- Law enforcement
- Quality/code enforcement
But let's ignore all that. We'll stick with just 1-7.
You can build the landing strip and the launch pad separately. In fact, it might make sense to have two landing strips separated by a considerable distance just in case the weather makes using one too problematic. Trade ships have schedules and they won't always be capable of waiting for the storm to pass.
You might also want to consider building both facilities near large rivers or oceans to include naval ports and land-based terminals so that goods can be delivered to and from the landing and launch areas. Your colony might not need them much right now, but they will need them later.
You know, that Harrier-style space ship that needs little more than a big square of reinforced concrete is starting to look really good.
The length of your landing strip depends on the mass and velocity of your incoming craft. As mass and velocity increase, the length of the landing strip increases. Note that you will need a way to get the landed craft over to the launch area and have the ability to manufacture rocket fuel and boosters, with means either recovering boosters left in space or building new boosters over and over. Now we need:
- Bauxite mining
- Bauxite processing to get aluminum
- Chemical processing
- Oxygen/hydrogen condensation
- Manufacturing facilities
- People, people, people....
Man, I'm really digging the Harrier-style space craft. Remember, this is the simple explanation.
Your launch pad will be deep. that's because you need to vent the launch exhaust. Unless your rockets can be tipped-up into place and stand on their own, they'll need a gantry system that allows for maintenance and crew/passenger access up until launch. You'll also need methods for preparing, moving, and storing goods and supplies for transport on the ship.
- Rocket assembly area
- Goods distribution area
- Some pretty big machines.
- People... people... people...
In a future where colonization becomes so routine that the idea that only a dozen people can colonize a planet (oh, the legal wrangling that would incur!) you need to develop space craft that don't need landing strips or launch pads. If you insist on either or both, you won't have colonies with just a dozen people, because even after manufacturing is done, you still need to maintain those facilities and you're getting fuel from somewhere (the cost if importing it vs. the cost of manufacturing it...).
Think about introducing Harrier-style orbit-to-planet transports in your world. That way the big ships don't need to land. Even if everything they're carrying is destined for one location, that simply means multiple trips with the I-can-land-anywhere Harrier-style shuttles. And it frees the big ships to haul goods for multiple locations (economics!).
With that being said, all you're really looking for is a sandy or small-gravel rocky area that can be smoothed out and used until frequency of use (due to population increase) demands something more durable and more forgiving to forklifts.