For any Garmin device if you use a >32gb you have to make sure that its formatted to Fat32. By default (in windows anyway) anything larger than 32gb gets formatted as exFat which won't be recognised by the Garmin device so if you are using a larger card make sure its formatted as Fat32 or you'll be frustrated.
Fat32 limits you to a largest file size of 4gb so any mapset that you download can't exceed this limit so the >Garmin>gmapsupp file that you make for the area you want cannot exceed the 4gb value. This is also the reason that the Garmin Zumo 660 and Nav IV with only 4gb of internal storage (1.5gb taken up by the OS) could not download the whole of europe using Garmin express when the last few NT mapsets for europe exceeded 6.5gb.
In practical terms you are unlikely to need more than 4gb even for a tour of europe as you can transfer a "corridor" of map blocks using map install that cover the roads you have planned to be on. I suspect that many on here myself included toured much of europe this way when a Garmin 2610 came with a 256mb CF card and we thought we were jolly clever when we bought 2gb CF cards which you could get most of europe on
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