If you don't want it to stick out of the back of a GBM, get a miniSD device. The M3 MiniSD should sit flush, so perhaps 4saken is thinking of the "slim SD" which takes regular SD cards and does stick out slightly.
The microSD M3 and Supercard products fit only in DS Lites. The M3 Lite should come with a shell to make it fit in Phats and GBMs, but I've never used one myself. I don't know about the EZ-Flash series.
The microSD M3 and Supercard products fit only in DS Lites. The M3 Lite should come with a shell to make it fit in Phats and GBMs, but I've never used one myself. I don't know about the EZ-Flash series.