Is it a Sony format that you're starting with, or that you want? If the latter, and you're working in text, simply use a Word processor. Design a template based on the size of the screen of your Sony; paste in the text; then change the font size to 18pt. Under "file" find "properties" and enter the book's name and author (to show up on the reader), then save as RTF.
Creating templates is easy. Just design it, delete all text, then save as template (it's in the save as drop down box).
Simples. If you get confused, ask Clippy, or whichever Manager you use!
RTF works just fine in a Sony Reader, I use it frequently. You only have problems if there are paragraph breaks where they shouldn't be in the original text. (Often happens with P Gutenberg).