Per Garmin, the largest card they recommend is 8 GB, but a larger card won't hurt anything, it just may not work. The following cut and pasted from the official Garmin website:
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02/08/2013
As long as the automotive device has the latest version of software, the device can use the following maximum card size from the table below. Please use the WebUpdater program at the following link to download the latest device software version:
http://www.garmin.com/webupdaterUnit Series / Maximum Size of SD Cardnuvi series / 8GB 1
LIVE series / 8GB 1
zumo 400 & 500 series / 4GB
zumo 220, 350, 600, & 605 series / 8GB 1dezl series / 8GB 1
StreetPilot c500 series / 4GB
StreetPilot c300 series2 / 2GB
StreetPilot i-Series2 / 2GB
StreetPilot 7200 / 7500 / 2GB
GVN 52 and 53 / 8GB 1
KNA-G510 / 8GB 1
Depending on the model of the device, it may use either a full size SD card or a microSD card, please refer to its documentation for more information. When using a card with a larger capacity and when the files on the card are large in number and/or size, the device's ability to draw mapping may be slowed.
For OEM dealer devices, please refer to the series above that fits the device.
1 Using an SD card size larger than 8GB is not supported for these devices. However, using one will not harm the device but it may or may not work. Also, Garmin supports all speed ratings (including unmarked cards).
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Jerry