Several posters have hinted but perhaps not said clearly that the choice of the size of the SMB depends a lot upon the environment in which you will be diving. If I think there is a possibility that I will be lost in high seas with people looking for me, I want one big enough to be picked up on Google Earth. I can sit on the surface and fill that sucker up at my leisure. Another use is to provide a good ascent line without concerns about being found in heavy seas, diverting boat traffic, etc. In that case a smaller bag has the advantages of being easier to carry during the dive and being easier to deploy when needed.