With a sump/fuge bigger is definitely better! More water volume = a more stable system. Also, the larger the fuge, the easier it will be to house the equipment that you want to keep out of your display.
My sump/fuge for the 75 is a std 20 and the sump/fuge for the 29 is a std 10. If I had more space, they'd be larger. If you are running a skimmer, I really like the setup that Gary (ohioreef) has with the skimmer on 1 side, the return pump in the center chamber, and the fuge section in the opposite side. With this setup, you can set the water level to the level that works best for your skimmer and since the fuge is totally separate, you can keep flow thru your fuge chamber slow enough for your macro to serve their purpose. Since the fuge is not in-line with the skimmer, you are not skimming out the beneficial micro-organisms that live within the fuge, the majority of which can (and will) pass safely thru your return pump and into your main display. These "critters" provide the bio-diversity and supplemental food that you want/need (IMHO) to have a successful reef setup. I do not run skimmer so my returns dump directly into the fuge section of my sump/fuge providing a food source to the "critters" living there. I'm not sure if Gary periodically feeds anything in his fuge directly or if this is absolutely necessary. Maybe someone else can chime in on that particular topic.
As always... just my .02,
Mitch