Question 314242
You just have to worry about the diagonals of the rectangular pizza stone.
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Apply Pythagorean theorem:
Let x = diameter
then
x^2 = 16^2 + 15^2
x^2 = 256 + 225
x^2 = 481
x = 21.93 inches
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It will just barely fit.  Now the question is, are there any obstructions in the way -- such as, the sides of your grill or the grill itself (is it flat?).