Oh, I misunderstood your question. I thought you were asking about GBA support.
I believe badzman is correct, the CycloDS supports any microSD or microSDHC card, which can potentially go up to 32GB. There is currently no 32GB microSDHC card that I know of. I think 16MB is the largest available (and those are quite expensive).
In reality, you probably don't need more than 4GB unless you plan on putting every game made onto a single microSDHC card. A 2GB microSD card can be formatted with FAT16 which is a little faster than FAT32 which is required for cards greater than 4GB. Also, microSD cards use slightly less power than microSDHC cards.
To give you some idea of how many games a microSD or microSDHC card will hold, a 2GB microSD card holds the firmware plus around 30 or 40 games (depending on how large the games are). I personally have a 2GB Japanese Kingston microSD card because I don't play more than 30 games at once. I just put the games I want to play on there, and delete the games I've already finished (keep a backup of the save file on my PC in case I want to play the game again later).
-Bri