What is the largest microSD card that a Monoprice Select Mini can read?
Unfortunately, the answer isn't as simple as that a specific size of SD card works and another size doesn't. The Marlin firmware wiki mentions:


  The SD- or MMC- Card must be formatted as FAT and must have a MMC interface. This is more likely with cards &lt;= 2&nbsp;GB.


MMC is the predecessor of SD. SD cards are not necessarily fully backwards compatible with MMC. Apparently, Marlin uses some features specific to MMC, so your card should support it.

The SD card support in Marlin is based on the Arduino SD Card Library, which further mentions:


  The library supports FAT16 and FAT32 file systems on standard SD cards and SDHC cards.


Based on this:


Cards should be formatted FAT16 or FAT32.
Cards bigger than 32&nbsp;GB definitely won't work (not SD or SDHC).
Cards at most 2&nbsp;GB will probably work.
Cards between 2&nbsp;GB and 32&nbsp;GB might work, depending on the specifics of the card.