I'll make a note of this suggestion.
@Alicia, The ability to change distances is on the roadmap, and should be implimented down the road.
As an aside to this, An inch is an inch, and the only reason it's being displayed differently is zoom levels. One thing that would probably help with sizing maps is the resolution of the map (Not the physical dimensions, like 500x417 pixels) but the DPI. Dots per inch, or pixels per inch (depending on if it's being printed or displayed on the screen) are the deciding factor. In most cases, it'll be somewhere in the 70-75DPI range. However, if the map is scaled for printing, and the squares are truly 1:1" you can get a square count by dividing resolution by DPI
For example: in the attached map, if you count the suqares, it's 10 wide and 20 tall
Now, if you look at the resolution, it's 720x1440 pixels at 72DPI
Run those through your handy calculator, and we know that this map is scaled to be 10 inches wide, and 20 inches tall
Currently, a square in Roll20 = 5 feet, and objects are sized in feet, so I know that my 10x20" map needs to be 50x100 feet. Clicky dragy, Bob's your uncle. Map, meet grid.
If your map has a border, or padding that is NOT an inch, you can move the object free of the snap-to-grid feature while holding alt. Usually lining up the topmost and leftmost lines of the map will make sure that you're aligned properly, then you can resize away.
As a final disclaimer, and an end to this wall of text:
If the map was resized, or was never sized to be truly 1:1" then all this won't do you a lick of good. You could load it into an editor like GIMP or Photoshop and resize it yourself though.
Hope this helps, and I'll still make a note for the Devs that sizing the grid by pixels would be preferred.