Set dimensions of downloaded map to make them realistic

Hi everybody, when you download and use free maps you find online, how do you scale them to get realistic proportions? I mean, sometimes free stuff is intended to be used on Roll20 and VTT in general, so there is a "sample square" that help you to scale the whole image properly to make it good looking. But sometimes there is not point of reference, so I would like to know which method you adopt  to scale image properly.

A lot of times map squares are set to 70x70 pixels.

For maps in Roll20, I'd recommend either 70 or 140 per unit. BUT remember that you can have your GRID smaller than a Unit.  If you take a map that is 12 Units by 16 Units, you could for instance have 10ft Units and 0.5 Grids which means you actually have a 120ft by 160ft map with a 5ft grid.  And you could easily have 15ft Units and 0.3 Grids to make that same map become 180ft by 240ft. And so on.  It just makes the tokens get quite small below about 0.3 Grids.