I have Shure E2Cs with 3 pairs of foamies. One pair is obviously a large size but the other two pairs are virtually the same size and i can't be sure which pair are supposed to be small and which pair medium.

I had the same exact problem, just try comparing the thickness...or put on on top of one another.

Hmm so it seems that the large size are deeper and thicker while the other two pairs are about the same depth but slightly different thickness. It gets quite diificult to compare the two smaller pairs if one pair has been molded a bit by wearing them.

The medium sleeves are slightly longer than the small sleeves. They are also a little bit thicker.