I don't know much about VTTs but I have used Fantasy Grounds a bit. It seems to me that resolution is highly dependent upon the screen resolution you're running. On my laptop I use smaller maps than when I run it on my desktop...which would be a bit of a pain if I ran the same campaign across both machines. I have a *lot* more real estate on my desktop monitor.
If you wanted to standardize I'd say try to make it so the map can fit comfortably on a 800x600 (low-spec) or 1024x768 (high-spec) screen along with the other info you need (chat window, character sheet, menu bars, etc.) Of course, I listed old-school screen resolutions...widescreens (which many people have these days) can get a little more width.
One given that may help is that folks can always scale the image downward but scaling up is a problem...so I'd say bigger is generally better.
Don't know if that helps at all.