I don't think a wee screen will make that much of a difference.
I used to use a Dell PDA as my Sat Nav that had a 3.5" screen. It broke and I reverted to a Nokia N73 phone as my Sat Nav which is tiny compared to the Dell. After a couple of days of use I'd never have known the difference.
In fact I got the Dell repaired and then felt it had so much wasted space as a Sat Nav
