This is from this site, by Dave Tate...
"The top of the box should be 24x24. This will allow for one to sit way back without falling off the box.
"The best way to build the box is to actually build a series of boxes. The box can be made out of 2x4s set to equate the dimensions above. Set the 2x4s so they form a box. Use 3/4 inches plywood for the top. This will make a 4 inch box. Then all you have to do is duplicate the box 2 more times.
"You'll place one box down then put a rubber mat on top of it followed by the next box, then another mat. If you build the boxes this way and also have a 1 inch and 2 inch board, you'll be able to size any lifter.
"It's important to use the rubber mat to keep the boxes from sliding. This is how I'd recommend building the boxes. If you built just one box at 12 or 14 inches then you wouldn't be able to do any low box work. There are some movements we perform on a 6 to 8 inch box."