Dedicated car GPS device screens used to come in two sizes: 3.5-inches (diagonal) and 4.3 inches. I have long advocated spending just a little more for the 4.3-inch screen size, because that precious bit of extra screen real estate makes a significant difference in visibility and ease of use of touchscreen controls.
But thanks in part to competition from smartphones, and the desire on the part of GPS makers to differentiate their products in an increasingly competitive market, larger screens are becoming much more common. Screens in the 5-inch size used to be rarities, but now all of the big manufacturers have at least a few models. And some manufacturers, such as Magellan, have ventured into behemoth screen territory with 7-inch screens (the Magellan RoadMate 1700 with a 7-inch screen is shown at the top of the accompanying photo).
So what's the right screen size for you? While there are still a number of 3.5-inch screen size GPS models on the market, I no longer recommend them for any purpose. You can find 4.3-inch models for barely a few dollars more across the performance and features spectrum.
Screens in the currently most common 4.3-inch size are fine for the majority of users. The extra real estate provided by larger screens is nice, but not necessary for most purposes. And as manufacturers improve screen resolution, as Garmin and TomTom have done with their new glass capacitive touchscreen models (Garmin Nuvi 3790T, TomTom GO 2405), you get a much sharper, clearer image within the 4.3-inch screen size format.
So why scale up to a 5-inch or 7-inch screen? Larger vehicles, such as RVs and trucks often have windshields that are further away from the driver than passenger cars. Also, trucks and RVs often have larger windshields overall, permitting larger GPS devices without blocking view of the road. A big-screen GPS is easier to see in larger cabs. Some GPS makers offer large-screen, trucker and RV-specific models, such as the 5-inch screen Garmin Dezl. In addition to its large screen, the Dezl has a speaker capable of louder-than-typical volumes, and a number of big-rig specific routing features.
If you're still not sure which GPS screen size is right for you, stop by an electronics retailer (you may want to call first to find out which units are on display) and try and compare screen sizes.