A series (Millimeter Measurements)
A0 – 841x1189.
A1 – 594x841.
A2 – 420x594.
A3 – 297x420.
A4 – 210x297.
A5 – 148x210.
A6 – 105x148.
A7 – 74x105.
A8 – 52x74.
A9 – 37x52.
A10 – 26x34.

B series
B0 - 1000x1414.
B1 – 707x1000.
B2 – 500x707.
B3 – 353x500.
B4 – 250x353.
B5 – 176x250.
B6 – 125x176.
B7 – 88x125
B8 – 62x88.
B9 – 44x62.
B10 – 31x44.

C series
C0 – 917x1297.
C1 – 648x917.
C2 – 458x648.
C3 – 324x458.
C4 – 229x324.
C5 – 162x229.
C6 – 114x162. 
                                                                                                            (Source :- Internet)