We used Niftydrive with Sandisk 128gb UHS-I Micro SD Card and this config to allocate more space for Macbooks with small ssd drives. For 128gb and 256gb models this is a lot more space.
Save yourself the trouble and don't try to buy UHS-II cards since they don't have better performance vs UHS-I in Mac.
Please use Filevault encryption in your main hard-drive. This setup makes it easy to use Time machine backups to the contents of the SD card too.