32GB will work BUT you need to format it to FAT32.
Depending on what version of WIndows you have, you may need to get a third-party program or force it to format to FAT32.
Here’s one how-to.
Also, the Sansa firmware was made before there were such large storage cards, and it may not be able to show all the files on the card if you have a whole lot of small files.
That’s because there is a limit on the size of the database, which reads information from the files–Artist, Album, Title, etc.–and makes an index from them. It will show thousands of songs, and I have filled a 32GB card and had it show every song.
But…there is a character limit on the size of that index, and many people have run up against it. It’s not an exact number of files; every letter makes a difference, so if your Artist listing is Rolling Stones rather than The Rolling Stones you might be able to show a few more files.
You can see if you have that problem when you use the card because your listings (Album, Artist) will stop before the end of the alphabet.
If you do have too many songs for the Sansa to show, you can get different firmware at www.rockbox.org that can handle a much larger (possibly unlimited–I don’t know) database. Read up on it before installing–it sits alongside the Sansa firmware so you can switch between them.