I don't have any comics to try on my Kindle DXG, but I used a simple Microsoft Word file and filled a single page with text. I set the paper size to 5.0" wide by 7.4" high and put a 0.0" margin on all sides except he right side I used 0.1", otherwise it clipped the last letter on the right side. I then saved the file as a PDF. See the scrrenshot below. The text pretty much fills up the screen except for appx. 0.2" space around the edges, which I think is a Kindle thing. You can use the same size of paper but add comics instead.
If you experiment with the page size and margins you will probably get a bit more screen coverage.
[EDIT]I created a similar file but with an image.[/EDIT]
Summary for PDF page setup:
- Page width = 5.0"
- Page height = 7.4"
- Top margin = 0.0"
- Bottom margin = 0.0"
- Left margin = 0.0"
- Right margin = 0.1"
...