Personally I'd go with A4 for size convenience and I just think it looks nice as a collection, but with that being said you just never know with these things. You could get there and customers ask for bigger sizes, I always have this problem when selling products of any kind. Maybe try out both with less runs of A3 due to cost and what not, you can always gage how things go for the next show. So long as it all fits in with your work you'll be fine.
I think postcards are pretty Kool, I tend to pick them up whenever available at shows or exhibitions, so you could try those. Sticker packs are good too.