spell effective radius

I am currently a W/Mo, but am getting bored with the build. I was considering switching my secondary to mesmer just to liven it up a little. In researching the mesmer skills, some of the spells/enchants affect "target foe and all nearby foes. So my question is how close do foes have to be in order for these spells to affect them? Is it a matter of a few feet or what? How can you determine the radius of the spell area?