The easiest way to set a scale is to think in terms of travel for the "world"

This is helpful:
http://www.cartographersguild.com/showthread.php?t=1908

A horse with a cart can travel 15-20 miles per day easily.
A lightly loaded person can make 12 miles per day walking.
Canoing is 10-20 miles per day depending on river current.
A large sailing ship (like a tall ship) can average 10 miles per hour.

Use these to set the scale... i.e. how long do you want it to get from one area of the map to another?

-Rob A>