I do a lot of gaming using the Roll20 Virtual Tabletop. What I have found when making battle maps for use there is that 105px/sq is the best resolution. At 100% zoom, the table is 70 px/sq, but it gives you the ability to zoom in to 150%. If you want your map to not look fuzzy at that resolutions, make it 105 px/sq. You should also make your image dimensions (including any border) some multiple of your resolution per square. This makes getting the maps set up appropriately much easier. I would also recommend leaving off the grid; most VTT will render their own grid. This allows the user to decide to use hexes instead of squares if desired (or vise versa).