SlingStudio supports UHS-I or UHS-II SD cards with at least UHS Speed Class 3 (U3) or above.
The size (capacity) of the SD card can be whatever best suits your needs, as long as the speed class criteria are met.
New SD cards must either be pre-formatted or formatted with a computer before using. SlingStudio supports these disk formats:
Formatting a disk erases all files on that disk. Before formatting an SD card that's been used for other purposes, be sure to check that it does not contain any files that you need to keep.
Finally, be aware that it is not necessary to have an SD card inserted into your SlingStudio unit (or a hard disk attached) to stream live video. You only need storage media present if you're working with a project that requires actual storage.
For information about which SD cards are compatible with SlingStudio, take a look at the article What devices is SlingStudio compatible with? linked below.