Depends on how many roosting bars are in your configuration - basically, you want the poop board to extend a few inches beyond the roost that is furthest from the wall. I think 6" minimum, but really give yourself as much room as possible, to keep the "squirty" poops on the board instead of the...
I have poop boards covered in linoleum (left over from another project), with a 1" lip; I've tried sand, but just didn't care for it. Currently using pine pellets for horse stalls as the litter medium, where you pre-soak the pellets to break them down into sawdust. Not too dusty at first, but...