This is an example of using one of my large-scale maps in FoundryVTT & Roll20 Free.

In Foundry VTT - this particular map is set at 50p on an exported JPG file that's about 32MB. These file types and sizes are available for my Patrons (https://www.patreon.com/morvoldpress), and can easily slot into the system for really large-scale combat, social encounters and city exploring.

In Roll20 Free - I have a constraint of 5MB file size, so the resolution is going to be much worse, but it's still possible. Obviously the Roll20 Premium account allowing 10MB would make it much better.