Help me with a level for this spell

[MacMathan]

Concussive Blast
Wiz/Sor
Evocation
LV ?
Comps: VSM
Range: Medium
Duration: Inst.
Area: 30'Burst
Save: Ref 1/2, neg. Fort neg.
SR: Yes

Damage: 1d6 fire +1pt force/ 2 levels (max 10d6+10)
Reflex save for 1/2 damage.

If Ref. save is failed target is also knocked prone. (What do you think about size modfiers for the knockdown part?)

Fort (or should that be Wil?) save or blinded for 1d4+1 rounds

I am thinking either level 5 or 6 off hand.

Thanks for the help.

 

[Anabstercorian]

Raise the damage cap to 20d6+20 and set the level to eight.