You will be incredibly surprised at how small M2 memory cards really are... Imagine a 5p coin, but a little smaller, and you've roughly got the size of it!
I clicked this lovely little chip into my sony erricson k800i mobile phone (which I also got from play.com) and it worked instantly, giving me a lovely 2gb's of additional disk space.
One thing I'd like to point out though, is that the actual size on the memory card is not exactly 2gb. memory cards, and sticks always give you a little less than what they say they are. For instance, the one I got was 1920mb, which falls a little short of 2gb (2gb = 2048mb).