Verified answer

A4 is 210 by 297

A3 is 297 by 420

A2 is 420 by 594

A1 is 594 by 840

A0 is 640 by 1188

lengths are approximate and in mm

A2 is bigger.

A4 is 4 times bigger than A5

A3 is 4 times bigger than A4

A2 is 4 times bigger than A3

And so on.

Good luck finding a poster that big xD

A2 is bigger