I had to looks this up; but I knew it had to do with paper size...

Definition: 'A' size or 'A' series is a set of paper sizes established by the International Standards Organization (ISO) that ranges from 2A0 (the largest) to A7 (the smallest). The size of the paper goes down as the number goes up, and each is half the size of the previous e.g. two A4 sheets make up an A3 piece and two A5 sheets make up an A4 sheet.

'A' size -- size in millimetres -- approx inches
2A0 -- 1,189 x 1,682 mm -- 46,8 x 66,2 in
A0 -- 841 x 1,189 mm -- 33,1 x 46,8 in
A1 -- 594 x 841 mm -- 23,4 x 33,1 in
A2 -- 420 x 594 mm -- 16,5 x 23,4 in
A3 -- 297 x 420 mm -- 11,7 x 16,5 in
A4 -- 210 x 297 mm -- 8,3 x 11,7 in
A5 -- 148 x 210 mm -- 5,8 x 8,3 in
A6 -- 105 x 148 mm -- 4,1 x 5,8 in
A7 -- 74 x 105 mm -- 2,9 x 4,1 in