I don't have an addition, but I have a correction:
A4 A rectangle with has sides in the proportion of 1: root 2 - the length of the larger side of the A4 rectangle is equal to the length of the diagonal of a square whose edge length is that of the shorter side
This is wrong in several senses:
Grammar sense: "A rectangle with has sides" must be which has its sides
Definition sense: This definition is actually for a silver rectangle. An A4 is a specific sized Silver Rectangle
to add to that, A4 is based on A0, which is the size of silver rectangle which has an area of 1 square metre.
B0 is the silver rectangle which has one side equal to 1metre.
C and D rectangles are designed to be envelopes for the above, respectively.
so to get A4, first make a silver rectangle which has an area of 1 square metre, cut it in half perpendicular to the longer side, repeat on the resulting A1 rectangle, repeat on the resulting A2 rectangle, repeat on the resulting A3 rectangle, repeat once more. If you did it again you'd have A5 etc etc.
I assume many of you know this but i spotted the chance for the forum to have a complete definition of A,B,C and D sizes on it.