That was it. They had to be in 64 meg chunks, but I also recall that the Meridian allowed only 4 maps, so even if you had a 500 meg, or a gig card, you couldn't fill it up.
Does the eXplorist allow for more than 4 maps?
I know that one session of map creation on Mapsend, only allows 4 regions at a time to be uploaded to the GPS, but once you have the maps on the GPS, you can rename them I believe, then try loading 4 more onto the card. Id forgotten, but I believe that I created the regions for the Platinum I had, by not having the GPS connected to the computer, and I saved the Map regions to the hard drive then copy how many maps I wanted to the SD memory card. GPS should be able to handle multiple maps over a quantity of 4.
I have the Whole of Aus on 2 128 Meg cards in 48MB chunks. All you need is a card reader to rename them something usefull. I just heard from Magellan that the Meridian Color will read a 1GB card so I can fit the whole country and all the tracks and waypoints I want on it.