10 point is a good size on the 6" eink screen of the 505.
Code:
html2lrf --top-margin=0 --bottom-margin=0 --left-margin=0 --right-margin=0 --wordspace=1.1 --base-font-size=10 "name of the file"
That's what I would use to get no mragins and a 10 point base font size.