Not an expert. I do remember reading an Amazon.com review for a different brand of SD card, I think ADATA, and one guy said he tested the speed of different sizes, using some memory speed tester tool, and he found the smaller sized one to be faster (i think he test a 4gb versus a 16b), but he thought maybe it was the card itself or brand, not due to the larger size.
I would like to believe size wouldn't matter, but who knows. I've got a 1gb, 2gb,8gb kingston (class 4) and 8gb Transcend (class 6) they work fine with my Acekard2i and R4. The class rating says how fast the micro card would be, (higher the number the better), but I really haven't bother paying attention to it. Also the older microSD cards, pre-microSDHC, might have different speeds than the newer larger capacity microSDHC cards (4 gb and higher I think).