I've always used a very simple way of balancing my missions - if I like playing them myself, I'm satisfied. To say it differently; no matter how you create your missions, there will always be those that like them and those that don't.
If you always make them the way you see fit, you will also be able to improve them once you learn more of the game and the mission editor.
As for me; I like those 1-2h missions after a day at work, and those longer ones on Sunday mornings.
Miika