To get complete control over font size, margins, etc. you could use the eBook Publisher software from ETI (the maker of the EB-1150) to make your books. If you don't have it yet, it's a free download from here
After you import your doc/RTF/HTML file, you can examine and edit the odf file created by the software. Then you can preview the book and make any additional changes you want. After the .imp file is created, put it in the program directory for the librarian and upload it to the reader.
Others may recommend Book Designer. I'll let others help you there, since it's not my favorite solution.