I've played FPW in 25mm, 15mm and 10mm. I agree witht the 10mm suggestion, it's my favourite scale for this period as you can field bigger armies and use the artillery as the Prussians used it. They are easier to paint too. Both Pendraken and Magister Militum produce 10mm, both produce great figures so it's just a matter of taste.
I've had some great FPW games in the past so enjoy.