Probably Garmin would say 16GB, or maximum of 32GB anyway because cards larger than 32GB are usually formatted in exFAT which isn't recognised by Garmin devices.

In fact the physical size of the card itself is of no real importance. It's other attributes which must be respected, most important ones: the format of the card [FAT16 or FAT32 only], the size of an individual image file [under 4GiB for the Astro 320] and the total amount of map tiles present on the unit and card in all maps including basemap [probably 4,000 maximum for Astro 320].

Even a 128GB card can be reformatted from exFAT to FAT32 with 3rd party software.

Best not to simultaneously enable two or more maps covering the same area, or even if they only have some overlaps. The Astro has only 1.7GB available for on-board storage but some is already in use, i'm unsure of the OEM Basemap size.

Note: I've moved your post to an existing thread. Another interesting one is here: [Only registered and activated users can see links. ].