garmin edge memory card

I know the upper limit is 32GB for the micro SD card but what is the highest useful size to get? For my JOGLE I don't want to run out of space for either routes or the recorded data so, with OpenStreetMaps (~500MB I think) also being on the card can I get away with a 2GB or a 4GB card? or is it worth going for say an 8GB card?
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I have a 4Gb card containing the Garmin OS maps (2.5Gb) and the OSM GB maps with contours (346Mb) and have 888Mb spare.
So a 2 or 4Gb Class2 card will do easily, no need to waste money on an 8G one...
It does seem that the historical data that Garmins store is impressively small for the amount of information contained.