From those dimensions I'm guessing its about a 50-60ish gallon sump which is a pretty decent volume sump for your system. It will do the job well but some with tanks that size, people tend to prefer sumps that most snugly fit under their stands for the extra water volume. Thats the way I see it, if you got the room under the stand, might as well make it useful to the system. But your sump would be fine, I mean that would be almost 300 gallons of water lol, pretty stable system if you ask me. If I were you, the last section of the sump with the skimmer on the outside would be my place for any supplemental equipment you have to hide it from your main display, you can even install a permanent plumbing setup for easy water changes (kind of just a powerhead with tubing/pvc with a cord unplugged until you want to do a water change then just plug it in). Be careful with the bioballs though, do some searching on this forum, you will see there are pro's and con's to them, pro's being surface area for bacterial growth and con's for being notorious detritous trappers and nitrate producers if not cleaned frequently (just the basic argument). Good Luck!!
-Josh
