look for the file
src\calibre\customize\profiles.py
Scroll down to around line 276 where you will see
Code:
class SonyReader300Output(SonyReaderOutput):
author = 'John Schember'
name = 'Sony Reader 300'
short_name = 'sony300'
description = _('This profile is intended for the SONY PRS-300.')
dpi = 200
Change that to
Code:
class SonyReader300Output(SonyReaderOutput):
author = 'John Schember'
name = 'Sony Reader 300'
short_name = 'sony300'
description = _('This profile is intended for the SONY PRS-300.')
dpi = 200
screen_size = comic_screen_size = (350, 350)
where you adjust (350, 350) to the result of your experiments.