One Odd-Sized page in Pages Document for PDF Need to embed a Numbers spreadsheet in a Pages doc that will “live” as a PDF. Generating a scaled image that I then embed results in mediocre resolution.
If there's no way to scale your numbers doc small enough to fit on a pages sized page, you'll have to use a larger paper size (e.g. legal). 

If you only want your numbers doc to be a different size, you'll need to handle them separately. I'm not aware of any way to change the dimensions of a single page in pages directly but... 

If the document will "live" as a PDF, why not export the pages doc to PDF in standard Letter or A4 size without your spreadsheet.

Then create a custom page size in your Numbers doc to fit the spreadsheet as a whole and export that to pdf.

Finally, use preview to combine the two documents.