I've fallen for the B5 size. It sits between A4 (which I find a bit too big, especially when a cover is added) and A5, which is on the small side. B5 is comfortable to handle and not too big to write with on my lap without a table or counter top.

I had followed a lot of reviews of various notebooks, but in the end I decided that an inexpensive B5 suited me better than anything else. For anything more compact, I use a cheap notebook and transfer to the larger B5 later.