Hi Tom Thanks that's a big help.As you know, the Nav V is a Garmin zumo 590. According to Garmin, all the current (not 400 or 500/550 series) zumo series, including the 590, can accept a 32 Gigabyte card. All speeds are supported.
See here: What is the largest size Secure Digital (SD) card that an automotive device can accept?
I have heard that some users are having success with 64Gb cards, but that is over what Garmin says will work. If the unit will not read it correctly Garmin will not stand behind the problem.
If you use a 64Gb card and have a problem, proper formatting may resolve the issue. Rather than go into that, I would refer you to this: SD Memory Card Capabilities for Navigator V - BMWSportTouring Forums.
Remember that "being able to read a card" and "being able to read the card in a reasonable period of time" do not necessarily go hand-in-hand.
I believe that the Nav V has 16Gb of internal storage. At present that is enough to store the 2015.3 North American maps with a half Gig left over. If that remains stable, then the SD card need be only big enough to store your music and perhaps routes (though a half Gig is quite enough for route storage on the device). Deleting unnecessary voice files from the device is not a bad memory management technique, as most of us would only want one or two voices in one or two languages; it frees up a bit of internal memory.