When you scan a letter-size (8.5" x 11") document using Acrobat, it produces a legal-size (8.5" x 14") PDF.
Some scanners don't report "supported sizes" to Acrobat when Hide Scanner's Native Interface is selected in Scanner Options in Acrobat. If this issue happens, switch the User Interface option to Show Scanner's Native Interface and select the page size from the scanner application.
The Scanner's native interface is disabled for WIA drivers. To enable the scanner's native interface, download the TWAIN drivers of the scanner, from the scanner manufacturer's website. Once you install the TWAIN drivers, the scanner options like DPI, color, and paper size are enabled by default.