More satnav thoughts

My satnav has a 3" screen and is now five or six years old, bulky and heavy, and it would cost considerably more to update the maps than the thing originally cost me. Meanwhile technology has moved on a lot, and there are several vendors offering fairly cheap 4.3", 5" and 7" units - basically Windows CE tablets with GPS and some sort of mapping suite. I know that you can do similar things with e.g. an iphone or Android tablet, but I really don't want to go that route.

I think 5" is about as large as I can really consider on a bike, even that might be pushing things. Does anyone have any recommendations?

later - to clarify this a bit, I use satnav a few times a year - I really can't justify spending a fortune, but I would like a bigger screen if I can manage to do it cheaply.