I'm trying to use the Foundry VTT and it takes an exact pixel value for tile size, along with taking a "scene dimensions" which by default is the image's resolution, but I just ended up turning off the grid here and using the built-in grid system for foundry, but I also ended up making the grid be rather small since I couldn't find a "good" size that would work with the terrain that also wouldn't end up being small.
Besides not being able to figure out the grid, (either square or hexagonal,) this is a great tool.