Choose a sheet or envelope size for its physical dimensions and area. This is a size reference, not a list of media approved for your printer. US size labels and A-series labels can refer to noticeably different sheets.
Reference basis
Letter is 8.5 × 11 inches; Legal is 8.5 × 14; Tabloid is 11 × 17. A4 is nominally 210 × 297 mm; A5 is 148 × 210; A6 is 105 × 148. The reference includes common photo sheets and #10 and DL envelopes. Inch-based dimensions convert with exactly 25.4 mm per inch.
Check three distinct dimensions
First, the physical stock. Second, the supported feed size for the chosen tray and mode. Third, the printable area inside the sheet. For scanning, use the original-size and scan-area limits instead. A printer accepting Legal paper does not prove its glass captures a Legal original.
The displayed converted figures are rounded. Manufacturers may also round dimensions in manuals, so use the named size and official product definition when interpreting a feed table. For an arbitrary custom sheet, use the converter. For document-to-stock mismatches, read the paper-size guide. The exact unit conversion follows NIST.