+1 on the very biggest you can afford/fit.
Just keep into mind that you have to think about maintainence when going with a larger sump. While you may be able to fit it in your stand. You may not be able to complete routine or basic maint. on it or the items in it.
Also while the door openings may be large. test fit test fit test fit. the sump, the fittings and plumbing, all of it.
If you are methodical in your planning then it will all work out. DYI sumps are not complicated, just do the research. Look at what other people have in there systems that is similar to your goal or what your trying to accomplish. Look on this site as well as others. Ask lots of questions like what people have found that works and why. Also what hasnt worked why. Look to people who are seasoned veterans of reefkeeping who have been doing this for years and years unlike my self.
You can never go wrong with a unit from Saphire Aquatics. The money and time you save doing it yourself may be well spent having a unit professionally built since thats what they specialize in.
What is your time worth?
I will be hiring them to make my next big sump.
Good luck and let us know how it goes