Sorry Tony, think your sizes are wrong!
The easiest way to figure it out is just put two sheets of say A5 together
in portrait mode and you get an A4, two A4s ditto = A3 and so on up to A0
An A4 in portrait is 210mm wide X 297mm high, so as an A3 = 2 A4's in
portait side by side, this gives an A3 in landscape which is therefore
420mm wide by 297mm high. Rotate your new A3 to portait, bung another next
to it, and ya got A2. There ya go, clear as mud!
HTH