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Just a little afterthought...
When you want you XPP page sizes to match exactly when viewed in Acrobat, you can always enter things in inches.
To take the same example of 51 picas x 66 picas, just divide by 6 and enter things as 8.5 inch x 11 inch and that way you will see exactly the same size in Acrobat (51 x 66 pica)
(by the way: the same goes for all measurements that you enter in 'mm')