For the Maxisys tool line it is a 32 gig, Class 10, Sandisk Ultra.  They are under $9 on Amazon

 I'm not sure , but The size limit of the external SD card likely depends on the size of the unit's Internal flash drive. I imagine the newer units with the larger 64gb  internal flash size  might be able to support a 64 gb Sandisk Ultra class 10 micro SD card. I would put one in my Maxi Sys Elite with 64gb internal flash ,to put my car history to but I'm afraid that updates by default would place BMW on the external SD card just like it does by default on the units that have the smaller 32 gb internal flash with space limitation.