As mentioned decided what the tallest thing you will be bringing in and add some relax space above. Doors can be bought in 3" increaments. 9'"3,9'6",9'9",10 etc. Anything over 8 is considered commercial. You will use the same opener on a 9' door as a 10'. 15" headroom above doors for up to 8' and 18" for anything above 8'.
If your door is going to be over the hoist that will be your height limiter. If you can, use a high lift track to raise your door as high as you can. This will mean that you will have to use a jackshaft opener. It may mean the difference between being able to raise your SUV all the way up or not. Don't forget to thicken the slab where your hoist will bolt down.
The size of building should always be decided by what is going in and what you will be doing in it. Measure it out on the ground and park everything in that space. Use cardboard or what ever is handy for tool boxes, bench's, etc. Walk around and adjust space as needed. Wall height based on how tall is the tallest vehicle you will put on the lift and how high you need to lift it to walk underneath. Don't forget room for lights.
Great thoughts posted about lumber, codes, etc.
Good to see your planning ahead. Have fun.
