How big is an a3 piece of paper in cm?

A5 paper is:

148 × 210 mm

14.8 cm by 21.0 cm

5.83" X 8.27"

(notepad size).

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Take a piece of A4 paper (the size that goes into a normal computer printer) and fold this in half so that the two shortest edges are together. The folded piece of paper is A5 in size.

Yes half of A4 is A5 but the photocopy or printer paper used in USA & Canada is not A4 size but 8.5x11".