The thing for the roleplaying VTT encounter maps is size. You want to activate the map and send it to players quickly so that you can get on with the encounter. Under 1Mb in size is good as it downloads reasonably quickly. A scale of between 30 and 70ppi to 5' is fine.
With the wargame VTTs, you are more willing to load a large map as it's all done at the start of the game.