The page about exporting: https://dcpomatic.com/manual/html/ch09.html# is short, I propose to replace this paragrapgh as follows:

"From this dialogue you can select the required output format, output file and, in the case of MP4, the quality of the output file by setting the CRF value*. The time needed to export and the resulting size depends among others on the resolution of the DCP (to change it, see here**) and on the CRF value. Reasonable CRF values CRF start at 17, but the resulting file size can be almost equal to the input itself. For non-distribution purposes, 28 or even lower can be a much saner choice that still offer good image quality."

*"CRF Value" should point to https://trac.ffmpeg.org/wiki/Encode/H.264#crf
** "here" should point to https://dcpomatic.com/manual/html/ch07.html