I'd say no, just by common sense, though by RAW it seems to be one size fits all. Custom fit is basically like custom tailored clothes of today - they start at a common size and then are adjusted to fit perfectly (or may be commissioned to be built to specific measurements). This is reflected in R&G like MijRal says - the penalty for wearing it is that you lose the Social bonuses normally conferred because those it is meant to impress are not impressed by your poorly tailored gear (or your being "short for a Stormtrooper" in the case of intimidation bonuses).
All other gear (including Custom Fit) has a size just like anything you would buy off the shelf. If it is within a size or so of yours, then you can wear it "right off the body", so to speak, though if it isn't your exact size, there should be a penalty (that is, of course, a house rule since the books do not mention it). Again, this is potentially needless book-keeping, so you are free to handwave it as a GM (same Stormtrooper reference - the trope where any character in a movie or TV show just happens to fit into the clothing of the guards they knocked out).
Or you can just do a random roll on the spot to see what size it is, say 2d6. A 6-8 means it is average for that metatype (or the same size as the character unless the character has specifically described himself as larger or smaller than "normal"), with each point off from 6-8 being one size too large or small. If it is one size too big/small, you suffer a -1 to Agi. If it is 2 sizes too big, you suffer -1 to Agi and Rea, increasing by 1 for each size. If it is two sizes too small, you suffer -2 to Agi and Rea, and you cannot fit into anything 3+ sizes too small. This gives a good chance (almost 50%) that "normal" sized characters can loot gear without penalty. If you want looting to be more difficult, then require a roll of 7 for "average" sized gear. Following on, for a nominal cost (this should not be a normal lifestyle expense as one usually buys clothes in their size and does not subsequently pay to have the fitted unless they are at least High lifestyle) they can have the items increased one size or decreased two sizes. I'm not a tailor, so maybe there is more leeway in fitting clothing, but since this is likely to be armor clothing, I feel justified in taking the conservative route.
Again, all house rule - by the book, it really seems to be one size fits all so long as you are the same meta-type, other than custom fit. Even though Orks don't pay a premium (they simply wear large clothes, but not abnormally so), I think I'd still say that an Ork couldn't wear clothing looted from a human or elf (and vice versa), though humans and elves are largely similarly sized so I'd allow them to share.
So, neither option above requires additional book-keeping since the roll can be done at the time of looting, though if it doesn't fit then it can require additional book-keeping if the character chooses to take the stat hit to wear it, but only short term and only if the gear is looted (you don't have to choose a size in advance when generating mooks). But, the additional complexity is on the player, not the GM.
