When I had a 2009 (192) RNS-E I was using the Transcend 4GB Cards. Older Nintendo branded SD cards will work also.
If I remember correctly 4GB is the max size and they must be non SDHC. There is a limit to the number of "total" folders (including sub folders) which the unit would understand, so you need to be careful with your organisation. Sorry, cannot remember the limitation figure. There is also a limit on both the total number, individual and combined, of characters for file naming and file path, like old DOS systems it has a maximum pathname length of 64 characters (pathname = folder name(s) plus filename).
ID3 Tags are limited to v2 or 2.2 as well, I used to set all to v1 to be honest and never had any issues. If you’re on a Mac, show all and then remove all hidden files eg the .ds_store, the RNS-E's get confused by these. cards need to be FAT32 formatted.
I have a 2010 (193) unit now and am using SDHC 8GB cards, believe the max is 16GB per slot.