Or more accurately, the earlier ones will take ANY SIZE of SD card, but don't recognize SDHC cards.
Since the memory address limitation on non-HC cards is 4GB, this means that you'll only ever find them in 4GB and lower... and usually 4GB cards are HC, so the most common sizes you'll find are 2GB and smaller.
However, since there's a limit to the number of files/folders that the RNS-E can recognise/count (I forget what it is at the moment) if your music files are all 128kb/s and 3-4 minutes long, you're likely to run into the file-folder limit way before you reach 2GB.
For that reason (and the fact that 128kbps sounds like steaming shi'ite to me!) I use 4GB cards and record everything at 320kbps (on my Mk1 RNS-E) and that just about meets the limit. -If I used lower sample rates, I'd hit the file/folder limit sooner.
SDHC adds more memory address lines, so can go to a larger capacity. -I understand that with the newer ones there may be a higher file/folder restriction... You'll have to read the manual to find out the details though, since I don't have a Mk2.
Keef