According to the article here: http://kindleformatting.com/blog/2009_06_01_archive.php
, the usable K2 book space is 520px x 622px. I think they are talking about books though and not pdfs, but maybe try those specs and see how they look. I don't have a K2 but on my DX I don't think any of the width of the screen is wasted in pdf mode, only some of the height at the top and the bottom of the screen is reserved.
If you are looking for an image processor you can try my manga app: https://www.mobileread.com/forums/showthread.php?t=75628
. You can specify width, height, dpi, color mode, zip output (.zip or .cbz), and pdf output. Right now it only works in batch mode, so if you want to process a single folder "X" you would have to place it as a subfolder in a parent folder "Y" and then run the program. In my next update to the program I will add single folder mode.