Feature Idea: Add a list or dictionary with valid 'page-size' values

It might be nice to have a list or dictionary with valid values for the 'page-size'.  Such a dictionary could have measurements or a description of the page size measurements.

This data structure could be derived from `pageSizeMap()` function in https://github.com/wkhtmltopdf/wkhtmltopdf/blob/master/src/lib/pdfsettings.cc