The best guide for most readers is to not choose a fixed format.
There's a strong contingent here, or at least there was, that does not consider any fixed layout to be appropriate for an eBook. I use any one of 4 devices for reading, with resolutions ranging from 1920x1200 to 480x320 - the other two are 1024x600 and 854x480. I also have access to a fifth device, resolution 800x600. Not only do the resolutions differ, but so do the aspect ratios - ranging from 1.33 to 1.78. They also range from 163 to 265 pixels per inch.
There's no one fixed layout that will work on all those screens. It's just not doable.