The first thing I need to note is that the only European country larger than texas or alaska is russia. Yeah... In fact, northern New England (from about Boston to the Maine-Canada border) is about as big, distance-wise, as England. Belgium is roughly the size of Vermont and New Hampshire squashed together. It bears noting that my home region of New England has some of the smallest states in the union.
now, as for what I do... hm. In Hero's Path, distance gets a little weird because of the Gates. If you've read the fic, as I know Glover has, you'd understand. But when Cole does travel overland (or flies...), I tend to make each city about a week's travel on foot from the other (give or take). There is one instance in League of Heroes where Cole claims that his Charizard has made the run between Cianwood City and Lavender Town in three hours. He admits it was flying at top speed, with a tailwind most of the way, but it also sort of marks that distance; the entirety of the Tojoh landmass in my fic can be traversed in a straight shot by a large, powerful flying creature in a few hours in the upper air, though it takes considerably longer on foot.
As for the size of Sinnoh... well, I'll use this example. It takes Cole roughly a week to get from Mt. Coronet to Snowpoint City, but that's on foot, forging through snowfields. In an earlier part, he flies from Canalave City to Oreburgh in two hours, pacing himself. In Unova, he can fly between Castelia City and Undella Town in a couple hours, roughly the time it would take for a small aircraft (like a cesna) flying from NYC to Boston.
Finally, the region I'm writing in now is about the size of Ireland, and noncanonical. So in closing... yeah, I'd say each region is about the size of main EU nations, like England or France.