I've been using a 12" squeegee since I started, just because that's what B&Q had in stock at the time, but I got 6", 10" and 16" ones this week to experiment with.
I'm finding the 10" one is best for most of my windows - obviously it needs more passes but it's easier and faster to handle on the smaller windows, narrow bay windows, glazed door panels, etc.
What's your preferred size?