What paper sizes support automatic 2-sided (duplex) printing?
Auto 2-sided printing is supported for A4, LGL, LTR, OFICIO, B-OFICIO, M-OFICIO, FLS, Indian LGL, F4A, and custom paper sizes (210.0 x 279.4 mm to 216.0 x 355.6 mm). Smaller sizes such as B5, A5, A6, and envelopes do not support auto 2-sided printing. (Page 9)
From the official Canon LBP1238 Ii Printer manual, page 9
Have a follow-up question?
Answers from the official manual.
Common questions