Usually the 5X7 for most trades....sometimes they cut them in half and tape them for smaller trades.
Don't buy very many....once you get started you will be able to recycle the envelopes that come to you.

Excellent suggestion-I even recycle ones that my relatives send discs on-what about a plain envelope for tiny seed-but with bubble (fine)stuff lining it?That might cut costs for some.

art777 thats what I did last year when I first started. some traders only wanted 5x7 bubble. I didn't have the funds to go buy many so I had 5x7 manila and a roll of bubble wrap from Christmas. Cut bubble wrap just a little under 10x7, folded it over and put it in my vacuum sealer to seal bottom and one side and slide into envelope. Worked really well. didn't get any complaints. Still have some left this year.
Cathy