Well, for me it depends what I draw, but the sketchbooks I carry with me tend to be A5 or even smaller, i.e. something I can easily fit in my pockets, however if I want to do a detailed drawing or something like a comic page I usually do that at at least twice the size I intend to have it in the end, so that it looks better when reproduced. But I can handle drawing in small spaces (probably comes from all the practice of scribbling in the margins during classes and such *grin*).

Thanks for the advice. I think considering my rather sporadic drawing activities I'll try with an affordable, small tablet.