It's part of calibre, and it is defaulted to PRS-500 display size
-p PROFILE, --profile=PROFILE
Choose a profile for the device you are generating
this file for. The default is the SONY PRS-500 with a
screen size of 584x754 pixels. This is suitable for
any reader with the same screen size. Choices are
['prs500-landscape', 'prs500']