I built mine 6'x6' and the next one will be larger probably 7'x8'. Two people would be do-able in the 6'x6' but cramped,three people no way. Mine is on the point of a ridge on a slope so the downhill side is about 8' from the ground and the upper side about 7'. I built stairs to go up to mine and am glad that I did. I used tin for the roof and siding with 2" of foam insulation that was then covered with scrap pieces of paneling. The insulation really doesn't do much to hold the heat in when the windows are open but help with sound.
I thought for a long time about what to use for windows and finally used second hand storm windows but with my walls being 6' wide I could only use one window per wall leaving more of a blind spot at the corners than I would like...if I would have went with 7' then I could have used two windows and not had as large of blind spot. The blind spots really haven't been too much of an issue once I learned where to sit and keep moving my head from side to side.
As far as deer seeing me in the stand I only remember that happening one time and that was because when moving my chair I bumped into an old metal crate that I keep in the stand for storage and to use for an end table to set my coffee cup and stuff on.
For the window sills I used 2'x10's with them being flush outside and extending inside and put sandbags on them making a nice shooting position.
For the gents,I dug a posthole about 3' deep vertically in-line with the third from the top stair and ran a 2' pvc pipe about 2' into the posthole and up to about a foot below the handrail with a 2' to 4' adapter then a 6' length of 4' pvc and a cap on a chain finally putting about 2 1/2' of gravel in the posthole and capping with dirt. A place to get rid of all of that coffee without carrying a Gatoraid bottle around.
I know one fellow who's son is a contractor and borrowed his trencher to run an electric line to a stand in the woods behind his barn.
I use my tower for squirrel hunting too. Two days ago I sat in it with my 17HMR and long range headshot squirrels for supper.