#The Dungeon Masters Guide gives a few guidlines

 - If a spell is so good that a caster would want to use it all the
   time, it might be too powerful for its level.
   
 - A long duration or large area can make up for a lesser effect,
   depending on the spell.
   
 - Avoid spells that have very limited use, such as one that works only
   against good dragons. Though such a spell could exist in the world,
   few characters will bother to learn or prepare it unless they know in
   advance that doing so will be worthwhile. Make sure the spell fits
   with the identity of the class. 
   
 - Wizards and sorcerers don't typically have access to healing spells,
   for example, and adding a healing spell to the wizard class list
   would step on the cleric's turf.

And then there's a helpful chart for average damage by spell level.

Spell Damage
Spell Level	One Target	Multiple Targets
Cantrip	1d10	1d6
1st	2d10	2d6
2nd	3d10	4d6
3rd	5d10	6d6
4th	6d10	7d6
5th	8d10	8d6
6th	10d10	11d6
7th	11d10	12d6
8th	12d10	13d6
9th	15d10	14d6