The Lights can be found in the Object Window, under the WorldObjects tree. All of the spell lights have the prefix MagicLight. The ones ending in Hand are the lights used when you have the spell equiped. You'll want to edit the ones you'd like to change and increase the Radius. For reference, 128 units is roughly the height of the player. You can also play with the Falloff and Near Clip settings, as these can dramatically affect the apparent brightness of a light source without changing the radius.
You should also check Nexus for mods that already do what you want; I know there are several mods around that affect spell lighting, and one of them may well brighten spells to your satisfaction.