Here are the international standard ISO ‘A’ page sizes for your reference in millimeters:
4A0 – 1682 × 2378
2A0 – 1189 × 1682
A0 – 841 × 1189
A1 – 594 × 841
A2 – 420 × 594
A3 – 297 × 420
A4 – 210 × 297
A5 – 148 × 210
A6 – 105 × 148
A7 – 74 × 105
A8 – 52 × 74
A9 – 37 × 52
A10 – 26 × 37