Roll20 does a good job sizing small items automatically when they are dropped from the art library into the map editor but struggles with items that are larger like tile sets.
Often such tiles when dragged from the gallery are sized to 1 cell wide or similar. So to correct this the item needs to be adjusted to display correctly.
Like any adventure there is always a starting place. A local tavern, inn or town square. Someplace safe that you can gather your gear, find any rumors or tips on the adventures that await.
It is my hope that this blog and its posts will help you along in your mapping adventures.
The first step in building with any of the DungeonArts (DA) sets is sizing.