I found an italian meringue buttercream receipe which requires 6oz egg whites to be whisked to a meringue. I have egg white powder, how much of this do I use, is it still 6oz?

I believe 1 egg white is about 2 oz. so I would reconstitute the powdered egg white to equal 3 whites.

Actually, I think you're wrong about that. I believe 1 EGG is 2 oz, one ounce white and one ounce yolk.

yep, I think you're right............30 grams is generally what an egg white weighs and that's just a little over 1 oz!

Supposedly, 1 large egg with shell weighs approx 2 oz: 1 oz is white .5 oz is yolk and .5 oz is shell. That said, you would still want to weigh the whites as they really aren't that consistant in my experience. As for using powdered whites, I have no idea. You can actually use real egg whites, though because the hot sugar you add will actually bring them up to a safe tempurature effectively pasturizing them.
BTW, I love Italian buttercream, and Swiss buttercream, and French buttercream. For that matter any frosting made with butter makes my mouth do the happy dance.