What are C sizes? Marketing Team Updated August 09, 2018 11:04 Follow C sizes are paper sizes used for envelopes, designed to take ‘A’ sized paper. We offer C4 (324 x 299 mm), C5 (229 x 162mm) and DL (220 x 110mm) sized envelopes. Shop our envelopes range here. Comments 0 comments Article is closed for comments.