Kari, I'm thinking ahead--before the swap, not during. I have tons of clothes I could offer up in theory, but what I bring should primarily be based on what the other attendees can wear. At least, that's what seems to make sense. I know sizes aren't clear cut but it would at least be helpful to know the range.
Let's say everyone at the swap was in the 6-8-10-12 range (random example), and I'm a 0-2-4. That would help me rule out bringing anything in a 0 or 2, and focus on what I have in a 4, because some of the size 6 ladies might potentially be able to wear a 4.
I guess in my case it's a little simpler since I think Lisa, Bella, April, and I are all in the same range (there might be other potential swappers, but I don't know who they are).
Still, might be helpful in deciding what has the best chance of being picked up by someone else.