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Most GPSs seem to be 4.3" and I see the odd one is 7". Is the larger one too big ? With glasses I would have thought that the larger one would be better. Watchya think?

Does size matter? For screen size this really comes down to personal preference.
When driving you should be able to trust the unit to guide you by voice. The key is to enter your destination before you start driving. Look for a device that ‘speaks’ the street names in the instructions and you will quickly find that you are able to rely on what you hear and will only need to glance at the display for confirmation from time to time.
If you buy a quality branded product you will quickly learn to trust its instructions.