The dimensions of an average sheet for a printer, i.e. A4 format, are:
A4 sheet in cm: 21 x 29.7 cm.
A4 sheet in mm: 210 x 297 mm.
An A4 sheet of paper is most commonly used for printing and faxing documents. You can also develop photos in this format, which due to their size look nice framed and hung on the wall.