I have a 50x150 La Grise and I have found that this is the perfect size for me.
I've been using a 150x40 for some time now. It's perfect for holding in your hand for honing.
Nice thing about the smaller size (for me) is that it forces me to work slower and more deliberately.
Best thing about a smaller stone is, it will get it done and not break the bank lol!!
+1. Smaller is better.