8.5x11 letter "paper size" is missing

I'm using Adobe Acrobat Standard DC to scan documents from a Samsung M267 printer.  I've done this many times before but all of a sudden I have a problem.  For some reason the option for letter size does not appear in a huge list of sizes.  Here's the procedure:

• File
• Create
• PDF from Scanner
• Custom Scan
• this gives me the Custom Scan window
• then I chose my Samsung printer
• then I chose 'both sides'
• then I click Scan
• this gives me another window, 'Scan Using Default Scanner Name',
• at the top it says, "What do you want to scan?"
• I chose the paper source, Document Feeder
• I chose Grayscale
• I click on Paper Size but cannot find 8.5x11

thank you in advance for replying