Your blast can manifest in a globe with a diameter equal to your Magic in meters at the cost of a Magic point.
If you must always use this, and thus cannot use the blast power without spending a Magic point, the metablast schtick is free.
This uses the normal area effect rules.
Area Effect
Some attacks and effects inherently affect an area - they always attack all targets in the area, usually not discriminating between friends and foes. Such attacks do not get a ActionValue penalty for attacking several targets. Instead, you attack the first target at your full action value, and each succeeding target is attacked at Action Value -2, comulatively. Third target at -4, fourth at -6 and so on.
Unlike a normal multi-target action, your targets get to decide which of them is most vulnerable and affected first. Also, if somone decides to do a "defend other" action, the defeisive value thus generated applies to the whole area effect - this assumes a stunt like "covering fire" or "jumping on the grenade".
If targets are very densely packed, an attack bonus might be appropriate; +3 per 10 targets is about right.
If friends or bystanders are in the area, make a separate attack roll that applies only to them, as if you made two separate attacks - one against friends and bystanders, one against enemies.