Scaling is always a problem, but there have already been some good tips. Start with the real world. Imagine an area on earth that might be similar to what you want to show on your map and look up for distances there.

But basically I don't think there is anything wrong with the relative scale of your rivers or mountains. You have to consider that on the maps that used this graphical style, geological features were not up to scale. There were just symbolical representations of such features.