For GBA, really a 4-8gb card will suffice. Because of the miniscule size of games and the fact all MicroSDs will flash at the same speed because they are all fast, that's all you unless you are a complete collectamaniac or something like that.
Personally I have a 4gb card with about 150 GBA games and homebrew, and ~30 GB(C) roms thrown through Goomba Color's latest beta, and I still have half the card left over to spare.
Good tip though: Unless you are sure about the card you have ordered (White for MiniSD original, Black or Blue for MicroSD), BE SURE to get a spare 2gb or under non-HC SD card. Older firmwares don't recognize HC cards because 1.74 was before the microSD implementation, and White classic cards generally come with 1.72R or 1.74. USUALLY though Black/Blue come with 1.75+.
And on a side note... Please do skin any new carts you get. The defaults EZ Team use are... Erm... Yeah. Clearlooks exists for a reason.