Word document - layout size

The Size device on the Layout tab has been buggy since day one. My guess is that you just never had occasion to notice it before now :-)

If your default printer offers Borderless for certain Paper Sizes, that list will default to display of the Borderless option being highlighted regardless of which you choose. IOW, it does not accurately reflect the actual Paper Size... E.g., if you select US Letter, US Letter Borderless will be highlighted when you reopen the dropdown list. The option you selected, though, is what is recorded in the Page Setup dialog & is the true Paper Size specified for the document.

All of that having been said, it really makes no difference unless the document includes content which is positioned near the edge of the page. For standard text print jobs, virtually all printers impose minimum margin settings [which vary dependent on the specific printer]. The minimums are observed unless Borderless is selected. Borderless enable the printer to ignore them in order to print to the edge of the paper & typically is used for printing photos, drawings or other graphic documents. Any content extending outside those minimums is cut off when the document is printed to paper or PDF is Borderless isn't specified.

In short, that most likely is not the cause of the problem. Unfortunately, there isn't much to be seen in your screen shot - It's a blank document consisting of a single page, the Rulers aren't displayed, & the open dropdown list covers the most important segment of the page so that the non-printing characters can't be see [if they're even toggled on].

With nothing more to go on, the first suspicion is that the solution lies in the Format> Document - Layout settings: Check to make sure that the Page> Vertical Alignment is set to Top rather than something else.

Another possibility is what Ping Cheng attempted to point you to, but the link s/he provided contains yet another link. To get directly to the pertinent information please see: