blind build's ?

what is the ideal inside height? like for most people

I would think 7'on the high side and 6'6" on the low side.

May as well just go 8' save your self some cuts and just use the whole sheet of plywood. If you cut a foot off it's just going to go in the trash.

I built one of mine basically as an 8' sq box. Side exterior panels dropped to cover base plate though, so around 7'6" inside height (less purlin studs). Interior height can be whatever you want, factor in materials and what waste you can tolerate.