To give credit where credit is due: I became aware of this technique in the post at https://productforums.google.com/forum/#!topic/docs/m-hUu90ZGI0.
Google Docs is wonderful in many ways, however, there are a couple of areas where it has limited functionality that were issues for me recently and this is the first of a two-part series of blog posts (see the second part here) on what I did to work around those limitations.
Google Docs only allows you to create documents from a set of standard page sizes; it does not allow you to create a document of a non-standard page size (custom page size) of your choosing. Fortunately, there is a workaround that will allow you to create a document of any page size. If you're printing (in my case, I was printing envelopes), working from a document that properly represents the size of your print media is at the very least helpful, if not downright critical.
The workaround is to create a document outside Google Docs (yeah, that's bad but it's what you have to do) with the proper page size (and get your margins like you want them as well because if you try to change those in Google Docs, you'll also be forced to change the page size at the same time and of course you'll be forced to change the page size to one of the standard sizes). I had success uploading a Word-type document (both .doc and .docx worked for me) but not uploading a PDF or a Pages-type document. Note, however, if you don't have Word on your machine, you can get OpenOffice for free. In addition, if you have Pages, you can create the document there and export it as a Word-type document and that works (in fact, that's what I did; for more on how to do it, see this associated post of mine).
One special note here: I encountered a bit of a strange issue with pages with a width of 7.25" (my custom size also had a height of 5.25" but I didn't investigate whether or not that was relevant to the issue). The issue was that Google Docs for some reason was inserting a blank page between every actual page on print/download. Very weird stuff. By using a document with a width of 7.75" with an additional .25" margin on left and right, Google Docs did not insert that extra blank page and for my situation, this was a suitable workaround because what I needed to be able to do was print and on my Mac, the key for printing was the center of the content (if in your situation, you ever see an issue like this but the key for printing is the left of the content then add additional width to the page and instead of dividing the additional width evenly between the left and right margin, just add all the additional width to the right margin).