I think the final answer depends on what you are trying to do -- what your campaign environment is going to be like, and what kind of feel you want the world to have. That's not the physics/realism answer, but it's probably the narrative answer.

One thing you could do is reason by analogy. For example, I see places in your continent that are reminiscent of Iceland, or the Great Lakes in North America, or the Nile River. You could take a look at those places and see how big they are relative to your scale, and inform what scale this continent should be. Or, for a socio-political version of that, you could look for an area on Earth that has a similar number of countries/cities/ethnic groups to what you want to have.