I don't think the RS-MMC cards were ever made any bigger than 2GB.
I've personally never seen an RS-MMC bigger than 1GB though. The form factor has been long abandoned. Any available cards are going to be used or no-names.

I have a 1GB and best I could find a couple of years ago at a reasonable price was 512MB Sandisks. Still have a couple brand new.

Anything over 4GB would be SDHC anyway and unlikely supported by the hardware, even if you managed to modify a regular full size card.