Standard sizes are more cost effective as they make the most economical use of the paper:

Business Cards – 85mm x 55mm
Letterheads – A4 – 210 x 297mm
Compliments slips – DL – 210mm x 99mm
A7 – 74mm x 105mm
A6 – 105mm x 148mm
A5 – 148mm x 210mm
A4 – 210mm x 297mm
A3 – 297mm x 420mm
A2 – 420mm x 594mm
A1 – 594mm x 841mm
AO – 841mm x 1189mm