I agree with the advice here. The desired image size largely depends on which VTT you're using, too. I'd say between 50-100 pixels per square is good, and as long as it's a round number then players can resize it simply. MapTools easily has the most room for the map, and does well with 100 px/sq, while Fantasy Grounds 2 typically has the least dedicated map space, so I like 50 px/sq there. Roll20 is similar to MapTools, though a little more is covered up--but either way Roll20 defaults to 70 px/sq token-snapping.
Something you might do (especially with large maps) is offer an alternate version which drops the color count significantly to reduce file size. The Caves of Chaos (B2) map I'm making is 740'x500'. With a 5' grid that's 3700x2500 just at 50 px/sq! Granted, if you're making battle maps it's not as big a deal, but for a dungeon-roam where you want the whole map, you can save a lot of file-size by dropping the color palette without losing much on the image.
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