Here are the PDF page sizes you can choose from if you use a Custom theme:
US Letter
8.5" x 11.0"
21.6cm x 27.9cm
A4 Paper
8.27" x 11.69"
21.0cm x 29.7cm
Technical
7.0" x 9.1"
17.8cm x 23.1cm
US Trade
6.0" x 9.0"
15.2cm x 22.9cm
A5 Paper
5.83" x 8.27"
14.8cm x 21.0cm
Digest
5.5" x 8.0"
14.0cm x 20.3cm
Paperback
4.25" x 6.87"
10.8cm x 17.4cm