The one I posted has 1/8" peg board with 1/8" holes on 1" centers. No cross bracing under the peg board or any other bracing at all. It has not caved in on me and the shop vac produces enough suction that if I lay a solid, flat piece of wood 6" x 6" or greater the suction makes it hard to pick up. Some times I have to turn off the vac or slide the piece to one edge to flip it over.
Mine measures 12" x 23" OS dimensions, but I'm afraid the if you get too much larger or increase the hole size too much, you might loose some of the suction power. However, saw dust isn't all that heavy so it may not make that much difference anyway.
Also the size might make a difference on where you store it if you are going to make it movable. I just don't have all that much room in my shop if I did, I would make bigger table with drawers for sanders and paper underneath and put it on wheels.
Some of you guys go ahead and experiment with various sizes and materials. I would guess that 1/4" thick peg board with 1/4" holes and about 18" x 30" would be a good place to start. Maybe even perforated metal about 16 gauge would make a nice top?
Rog