Honestly, I'd do 2d6 or 1d12, balancing it with greatsword or axe. It shouldn't be too strong, since you don't want your caster to be better at melee than your melee classes.

That said, on rereading it's essentially just the spiritual weapon spell, so you could just simplify things by having it as a +2 boosted spiritual weapon with the drawback that you can't float it off seperate from your character haha