I've gotten the impression that the regions were more like.. small countries? Or maybe different regions to a larger country. I'm not all too sure.
But anyway, here's my view, for my story I found myself really curious as to least how big they actually were, and how long it would take to walk across the region like trainers normally do.
I went on Google Maps, and under the directions options there's one where you can get a route by walking. For example, in order to walk from Orlando, Florida to Atlanta Georgia, it would apparently take about six days. Onward from Atlanta, to Chicago, Illinois, would take about nine days.
Now I don't know how far apart or how long it actually takes to walk between the major cities. But I normally think of them about as far apart as major cities are here. In the anime, there are several smaller towns in between, which of course they could hardly include in the games. But that sounds about realistic, right? Cities like New York, Washington, Orlando.. they would be main destinations, most likely with a Gym. With scattered towns and smaller cities in between.
One more little detail, I read somewhere on some site that it apparently takes about a year to complete a single regional Pokemon league. Or at least that's some estimate based off obscure hints and details in the franchise. So then factor in walking time for.. wherever you're going. Of course, that could be off. Don't trust everything you read on a fan site, after all.
Okay guys, I'm done rambling now.