I have a older Garmin 60CSX and I have used microSD up to 32G on the GPS. I had suspected that the card size had something to do with the time it took to complete drawing of the map on the GPS. I tried a little experiment with a 8G, 16G and 32G cards which I had. I loaded the same a 3.5G mixed map (topo and street) gmapsupp.img file in the Garmin folder on each of the cards.
This is the time it took the GPS to complete drawing the map. All cards were ScanDisk cards.
8G (2) 65sec
16G (4) 58sec
32G Ultra (1) 35 sec
The results of the last card surrprised me. The number in brackets is the speed class of the card. It was my impression that the speed class was the number of MB/sec that you can WRITE to the card. It was also my impression that it had nothing to do with the speed of reading from the card. I repeated the test a number of times to verify the time recorded. I also thought that it might make a difference and shorten the time the second time the map was loaded. No difference was noted.

I also have used the 32G SanDisk Ultra (1) card on a Nuvi 855.

The large cards contain multiple IMG files and I use GarminMapSelect.exe program to rename the maspsets to suit my needs. In the case of the 60CSX only one file (4G limit) is recognised by the GPS.
In the case of the Nuvi the card also contains JCV. ASR and a lot of MP3 music files.