I bought a 2gig SD card as recommended on these forums to use with my RCD510, but when I try inserting it, it doesn't seem to quite fit properly

In that it's a little too small for the slot and could fall in if not careful. I thought this "type" of SD card only came in one size but clearly not!
So without fear of asking a stupid question, just which size (physical) SD card do I need? Or more to the point, what SD card are all you RCD510'ers using?
