As Lauren points out, it's best to ask the self-publishing service or printer you're using about the page sizes and binding options they provide. However, here are some more general page size considerations.
Obviously, one of the main ones is cost. Aside from having fewer/larger pages to save paper on margins, you can also cut costs by using page dimensions that your printer's press sheets will divide evenly into.
The size of the press sheets depends on the paper stock you choose and what your printer's presses can handle. You should discuss this with them after you've chosen a paper stock, but otherwise sheetfed paper usually comes in dimensions that are a multiple of the North American ANSI A 8.5x11" "letter" size (or the ISO A4 210x297mm size in other regions).
The most common page size for books is 6x9" (about 350 words per page). Other standard sizes that will be cheaper to print in North America include 5.5x8.5 (about 300 wpp) and 8.5x11.
If you want to do the page layouts yourself, I would recommend reading Grid systems in graphic design by Müller-Brockmann,¹ which is the seminal work on grid systems in typography and is a must-read. It covers everything from choosing font sizes, the effect of different line lengths, choosing leading for different types of work, paragraph spacing, choosing page and margin proportions, etc.
¹ - Grid systems in graphic design was first published in 1961, so it should have passed into the public domain in '89, but with all the stupid copyright extension laws that have since been passed, it's possible that the book won't be public domain until 2028. But I would still try to find a cheaper alternative before shelling out $75 for a new copy.