Hriston
Dungeon Master of Middle-earth
I've realized this answer is technically wrong, so I wanted to clarify. The above is how I run it for a medium or small sized caster. However, if the caster is larger, it does not increase the size of the spell's AoE which remains a 30' diameter sphere.I rule this affects creatures that could be attacked if the caster had a 15' reach, so a creature three squares away from the caster (two squares diagonally) is considered within range.
I believe the relevant rule is to be found under "Sphere" under "Areas of Effect" which states "A sphere's point of origin is included in the sphere's area of effect." So if the point of origin is the caster, as indicated by the spell's range of "Self", then the area inhabited by the caster is included in the 15' radius AoE.