You can't script the Page Setup or any of the popups in it. Well, you can if you use the UI scripting stuff, but that's a bitch to figure out and use. But you can do what you need, but you need to print in order for the size to "stick" in the document. Simply setting the paper size in the document won't do it. Sending the value along to the "print" command will force that value to be used. You can just print to pdf from there or anything else except canceling.
tell application "MultiAd Creator Pro"
activate
set syz to size of page 1 of document 1
set page options of document 1 to {page position:{horizontal position:at left, vertical position:at bottom}}
print document 1 print settings {custom paper size:syz, use custom paper size:true} with interaction
end tell
I recall that trying to work with the print manager had lots of roadblocks, and this might be one of them. This will also only work for documents that use a single page size, as it grabs the size of the first page. If it doesn't end up in the right location on the paper, you'll need to mess with the "page position" values in the "page options" property.
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Steve Mills
Drummer, Mac geek