You are probably used to seeing "A4" paper, commonly used for printers and writing paper. This is from the ISO standard, the most common system of paper (and similar media) sizes in Europe.
The aspect ratio of ISO paper sheets is 1:1.4 (the square root of 2), which gives them a unique property - if you cut a sheet into two, the resulting halves are the same proportion as the original.
If a sheet of A0 (which has an area of 1m2) is cut in half, you get two sheets of A1. Half of A1 is A2, half of A2 is A3, half of A3 is A4 - and they all have the same proportions:
| mm | inches |
A0 | 841 x 1189 | 33.1 x 46.8 |
A1 | 594 x 841 | 23.4 x 33.1 |
A2 | 420 x 594 | 16.5 x 23.4 |
A3 | 297 x 420 | 11.7 x 16.5 |
A4 | 210 x 297 | 8.3 x 11.7 |
Please note - loose canvases are these sizes, but the finished sizes are smaller for box canvases, as the canvas is wrapped around the frame.