Change the font size in the document before you send it to the printer.
You may not be able to control the font size in an attachment directly in the mail window, as the document may not be editable - and won't allow changing the size of the font in the attachment. You would have to experiment with that, by saving the attachment to your Mac, then opening it in some app that may let you edit the document, like Word, or TextEdit.
But, you could make the printed document larger when you are in the print window - usually by increasing the number in Scale above 100%. That makes the whole document larger, and could make it occupy more pages. Depends on how large you want to make it. That's something else to test on different documents that are typical for your emails.
Stupid question because I'm sure you have your reasons, but why would you even consider printing documents? Paperless is an option you've apparently selected because you're being sent receipts and statements as email attachments.
Please save some trees by saving your email and attachments as PDF files on your SSD/HDD. You can always print when a hard copy is absolutely necessary.