Custom page size (crop) in Acrobat not working

Hi all, I'm trying to re-size the pages of a PDF - all I want to do is change the page size from 5.43 x 8.5 in to 5.5 x 8.5 in. I don't want to scale any of the page content, I just want the page size itself to be a tiny bit wider.

The only way I know how to do this in Acrobat is to go to the crop pages/set page boxes dialogue and enter a custom page size, but when I do this, nothing happens. The page size remains the same as it ever was. Nothing I've tried makes it do anything. It works fine if I enter a fixed size, but entering a custom size has no effect whatever.

I must be doing something wrong or missing something, but what?!

Grateful for any help.