I thought it worthwhile posting my experience of the largest SDXC capacities that I've found work reliably with the various Sansa models that I have.
All using Rockbox V3.15. Going to the next size up in capacity results in unpredictable behaviour, Panic crashes and freeze crashes. All SDXC cards formatted FAT32.
Comments invited.
E200 series: 32 GB
Fuze V1 128 GB
Fuze V2 256 GB
Fuze Plus 256 GB
Clip Zip 256 GB