I'm just assuming you're going to build your own sump (as I am)...the maximum size that you can get in the cabinet and comfortably clearing the hinges on the doors is a 16" by 16" cube. I have a thread here (
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1596863) where I asked for some opinions on a cube sump design and ended up with some very good suggestions that I will be taking into consideration when (if) I design mine.
The biggest consideration to make is whether or not you want to take that step to drill your tank...some on the forum here have done it sucessfully and have great setups (check out ReefMaidens thread here: (
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1599870) and there is also a GREAT youtube video of a guy who has drilled his BioCube and built a sump for it here (
http://www.youtube.com/watch?v=SfQ3-unDRtg&feature=channel).
I mentioned a few times on this thread that I'm also considering (instead of drilling and using a sump) the CPR Aquatics AquaFuge 2 (
http://www.marinedepot.com/ps_ViewI...iums-Saltwater_Aquarium_Supplies~vendor~.html) The smallest one has dimensions that should fit the BioCube 29...but I haven't really found anyone else who has used this setup on a BioCube yet so I guess I would be the first if I do decide to do this
I'm still deciding on which way I want to go...drilling my tank and building a custom sump/fuge would cost more and may be more of a headache to get done but would probably add more stability in the long run (maybe?) versus running the AquaFuge (probably less expensive and fewer headaches..so I could invest more money into that nanocustoms HQI light upgrade I want so badly

) which would still give me the benefit of running a fuge where I could grow macroalgae for nutrient removal...decisions, decisions.
I calculated the dimensions of the sump in my thread above...I think I figured if you have about four inches between the top of the middle divider (8") and the top of the sump (12") that it would give you about 4.4 gallons of water to "play with"...that should be enough if you were to use the stock return...I don't think it would drain more that 4 gallons past that return...hopefully someone will correct me if I'm wrong here.