A Question of Scale -WIP

[AntiCognito]

Hello, everyone! As you can no doubt tell, I am new to these forums. I decided that it would be a fun challenge to attempt to create a fantasy world for a D&D campaign which I hope to run, and I quickly found that what I was attempting to do was well beyond my amateur skills. Then I stumbled upon this site, and found some very helpful tutorials, and now I am very pleased with the world that I see forming before myself. However, I have a fairly terrible knowledge of geography, and, as such, have a few questions. Because of some helpful information about plate tectonics, I think that I understand where I should put my mountains and valleys and such, but I don't know anything about scale.

I know that the Earth is around 24,000-25,000 miles around, and right now my document is 3,000 pixels wide, so, if I make each pixel 10 miles, it would be around the right size of a planet. But I'm not sure how big to make the mountain ranges, or the forests, cities, marshes, towns, plains, etc. I don't know anything about the average size of anything. Also, for those of you who play D&D, how much ground would the average party of adventurers be able to cross in a day's march?

Thanks for the help. If you want to see the world so far, here it is (I plan on adding more in the way of color at some point):