I have the 14.5" gi peel with 20" handle, I like the size for the Blackstone. I think the 16" would fit but it would be a tight, Norma did some 16" pizzas in her blackstone - take a look at her getting the blackstone from Cabelas thread, she might have talked about the size.
I have never tried it but getting a pizza out of the blackstone with a wood peel but it might be tough to scoop it up. If you don't get a gi peel you may also want to get a metal peel to take the pizza out of the oven. The GI eliminates the need for two peels. Talk to John if you ever decide to get a GI, he'll take care of you.