im new at this game so by no means am i an expert i probably shouldn't even be voicing an opinion yet but i've been in the process of organizing an overflow and sump for my 72g, and i think the options he's referring to are things like wether or not you want to keep a refugeium, some live rock or sand, with enough space you can keep most of your electronics down there pumps, protein skimmers and heaters. you can set your baffles up with more of them so the water passes through them in such a manner that you don't get as many micro bubbles(still haven't quite figured out why their bad), or so you can run certain filter media between the baffles. more space allows you the room to keep certain live stock down there you dnt want getting eaten in your main display, copepods and such. the volume of your sump adds to the total volume of your system as well, making changes happen that much slower therefore giving you that much more time to react. not to mention the bigger you go the less likely you've pigeon holed your self in the future i agree the more space the better. my sump is going to be 33g. in order to get it in my stupid stand(doors in the center 1/3 of the stand)i have to cut it and weld in steel bracing. super lame but a little extra work im willing to do to get an extra 10 g. any ways hope this helps.
ohh yeah i think you'll definitely want a protein skimmer. but don't be dumb lucky like me shop around a little ask about some good name brands/prices before you purchase. i would fill you in but i don't have that info yet. i bought an instant ocean(around 150$ i think) from the lfs not knowing anything about them and from what i can tell it seems to be doing its job i empty it once a week wether i have to or not and what i dump is gross so it must be doin the job. i also have an oceanic that came with my 33g sump haven't used it yet but the guy i bought it from said it works good(of course he would lol)but he seamed like a very knowledgeable guy