If you're working by comparison, I would do some maths and have a "scale bar" in mind. If your world is half the size of Earth, then your map is about 20 thousands kilometers wide at the equator. So, it means that you get an equivalence of 1 pixel = 4 km ; or maybe 1 pixel = 3 km if you want to add some waters on each side.

Knowing this, you could overla/compare with a map of Earth putted to the same scale. That the way I've worked some of my maps for other planets (Mars, Venus, etc...).