Most people build these things based on average building materials. 4'x8' sheets of plywood mean that you could build a 4'x4' platform out of a half sheet, and 6 pieces of 2'x4' out of the other half plus another sheet. 4 pieces go around as the skirt and a + in the middle to give extra support.
2' won't make waist hooping sail above the heads of most people. Just measure your hips to the floor and add 2'. But it could mean that shoulder tricks and such go over them. Naturally knee hooping is a bit out. Alternatively, you could build a 4' cube which would put your waist over most people's heads.