A paper size mismatch means the job's requested size or source differs from the loaded tray or printer setting. Check all three places before changing hardware.
Compare the three sizes
Read the size shown on the printer panel, inspect the loaded stack, then open the document's print dialog. US Letter is 8.5 × 11 inches; Legal is 8.5 × 14 inches. A4 is different and may trigger a prompt when the tray is set to Letter.
Correct the job
Select the size you actually loaded and the appropriate tray. For envelopes or labels, check the manual's supported feed path and orientation. Avoid accepting an automatic scale setting without checking the document layout.
If mismatch remains
Print a built-in page and inspect the exact on-device message. The printer may still remember an old tray size. Use its model-specific settings guide rather than guessing at a sensor fault.
Official references
Useful next steps
Found a mismatch? Request a correction with the public source URL and exact model, without passwords or full serial numbers.