There is already a 2.5" monthly charm swap going on. 2" charms are probably not the size a lot of us use but as each quilter is his or her own artist there is room to use all sorts of different sizes whether it is the norm.
IMHO concerning the swap process it would probably be less expensive and more convenient to have all the charms mailed to a central location swapped from there and mailed out to the participants. This of course will depend on the numbers of participates.
I say this because the participants would pay only one postage mailing to the central location and provide return postage to receive the swapped charms. Mailing the charms in one large envelope (preferably USPS flat rate) helps to contain postage cost and keep individual envelopes from getting crumpled in the mail.
I have been in both type swaps, mailing to a central location and paying postage both ways and sending individual envelopes and paying the postage separately. I find that you need to add extra support to the individual envelopes to keep them from getting eaten in the mail canceling machines which adds to the cost of postage per envelope. This has just been my experience.