Margins are way more important than you might think. They actually have a lot of jobs. They provide space for the readers fingers, make your text easier to read, they are space for readers to write notes, keeps your text out of the gutter, provide space for headers and footers.
So how big should a margin be? It depends on the size of your book. The centerfold will include the gutter depth as well, so it is a larger margin by 0.15 to 0.2 inches. The margin at the bottom of the page contains your page numbers and is typically a little larger than the margin at the top. You will want to look at different sizes of books and get an idea of what is standard for the size and genre you are publishing in.
Kindle Direct Publishing has a minimum margin of .25. This is really only so your text doesn’t end up in the gutter or off the side of the page during printing and cutting. You should not use a margin this small. There needs to be enough white space (negative space) on the page to make it easier to read.
Many self-published authors choose smaller margin sizes to make more text fit on a page because it is then cheaper to print the book which means higher royalties for the author or so it looks at first sight. The issue is you may sell less books because of this formatting fopaux.
There are other ways to make sure you get the most words on each page. The easiest is to try different typefaces (font). You can get a lot of variation even with the same size of font just because of the style you have chosen. Examples
The red hen clucked at her chicks, who peeped at the mouse scurrying off with their seeds. (Bahnschrift light)
The red hen clucked at her chicks, who peeped at the mouse scurrying off with their seeds. (Adobe Devanagari)
The red hen clucked at her chicks, who peeped at the mouse scurrying off with their seeds. (Flamingos)
The red hen clucked at her chicks, who peeped at the mouse scurrying off with their seeds. (Garamond)
The above four typefaces are all the same size (11). For most books you are looking for 30 to 35 lines of text per page.
Another important term you may see thrown around when discussing margins is the “bleed.” Bleed is the very very edge of a page. Typically picture books have a bleed. Novels, in most cases, do not have a bleed. With a picture book you want to make sure that the pictures go right to the edge of each page. The bleed edge is just slightly outside of the trim edge. The trim edge is where the page is cut. Having a bleed ensures that if the page shifts during cutting your picture still goes to the edge of the page.