Each size doubles the previous size smaller dimension (in inches):
A size = 8-1/2 x 11
B size = 11 x 17
C size = 17 x 22 << looked this one up
D size = 22 x 34
In case you cannot find what you want locally, several Amazon vendors have them. For example, 17" x 22" Quadrille Ruled 50-sheet pad for $18 to $22, plus shipping.