Here's some practical thinking material for you: What is the fastest mode of transport your characters have available? Humans traveling in a small group should be able to cover around 15 miles per day without overly tiring themselves. On horses they can make perhaps 30 miles in a day, but the horses will need rest after a certain number of days (I don't know horses very well, so I don't know the limits of their endurance).

What kind of time period do you anticipate your story covering? Pioneers on the Oregon Trail took about 5 months to cover the 2000 miles from St Louis to Oregon in ox-drawn wagons. Note that they had to cross two mountain ranges during that time, and it also included some time breaking trails.

Those pieces of information should give you an idea of what kind of scale you should look at for your primary mapping area.