Work up your material list and then plan your cuts. Most building materials come in 4’ or 8’ dimensions. Eight foot lumber is cheaper than ten foot lumber. You should be able to build an 8x12 for less money, with fewer cuts, and less waste material to get rid of. An 8x16 may not be a whole lot more expensive or take any longer to build than a 10x10. Don’t forget your roof, though. It’s on a slant so its slightly longer plus you want enough overhang to have ventilation openings up there and still keep rain out.
You might follow the link in my signature to get my opinion on how many birds you can fit in there. There is no magic number that fits all of us, we are all unique in our circumstances and management techniques.
I like having removable roosts. Here’s how I attached mine. There are different ways to do all of these things. Just pick the way that you like.
I’m a big believer in flexibility and simplicity. You sound like me, over-planning and overthinking everything. I find that when you put these plans into practice, they don’t always work out as planned. For example, if you put the pop door at ground level you have no flexibility without major revision if the sand/gravel doesn’t work out for you. If you raise it a foot, they can still use it just as well but you have designed in flexibility. If you are worried about baby chicks being able to get in or out, use a cinder block, pavers, bricks, or build something as steps.
I think you will find that chicken wire under the roosts to be a royal pain and it won’t really gain you anything. It won’t hurt them to step on that droppings board, if they do much at all.
