As I have recently done some experiments and recorded the settings, I thought I would post the findings.
The font size range from 12 to 58. The default is between 22 and 25. I didn't try all the settings. I think I did the ten smallest and then a random lot in the larger sizes. Not all the numbers are there. All are the from 12 to 22. And then it skips one every few steps. From 40 to 58 I think it skips every second number.
The line spacing has the following values: 1, 1.35, 1.4, 1.6, 1.775, 1.9, 2, 2.2, 3. I have no idea why they would choose those but I haven't tried any other numbers. I thought they might change with font face or size, but they don't seem to.
The margins start with 0 and go up by 3 to 15. You can set this to any number in that range and it will be used. You might be able to set it to a larger number, but I haven't tried. And I can't see any reason to go as high as 15.
The justification is "off", "left" and "justify". "right" doesn't seem to works, but I haven't checked the styles of the book I tried to make sure that wasn't blocking it.
After a lot of playing, I find I like Georgia, font size 17, spacing of 1, margins 1 and justification off.
As an arrogant programmer (I'd like to think I am not typical, but I probably am), I have made decisions about this sort of thing plenty of times. But someone has to make the decisions. Hopefully an expert tells you what is needed. Sometimes you are the expert. Sometimes no-one has a clue so you make the decision and hope you get it right.