There are two systems in metric system or british system for all sizes products:
British system specification (inch)
metric system specification(cm)
5'×7'
13cm×18cm
8'×10'
15cm×40cm
10'×12'
18cm×24cm
11'×14'
20cm×40cm
12'×15'
24cm×30cm
14'×14'
30cm×35cm
14'×17'
30cm×40cm
7'×17'
35cm×35cm
35cm×43cm