Yes, I do know what card fits.
If it feels good, put in a 16GB Class 10 or higher card. I prefer Sandisk. That will hold the entire load of North American and Western European maps, but .. unless/until you start running out of the 16GB of space you already have, there's no point. If you don't plan to take your unit out of North America, and with the maps the size they are now, even if you DO, you won't run out of space using only the internal memory that you have.