When dealing with the EPUB format there are number of ways to deal with sizes. Font size, indent, margin, spacing all allow for a variety of units to define their size. Sizes can be defined using any of the following: %, in, cm, mm, em, ex, pt, pc, px.
With all these choices it might be hard to decide which unit type to use. This decision is actually very easy. Alway use a relative size type. cm and in for instance are fixed size; 1 cm is always 1 cm and 1 in is always 1 in. % and em are relative sizes. An em is equivalent to the the current font size. So 1 em is equivalent to 12 pt if the font size is 12 pt.
It’s very important to use relative sizes because EPUB and EPUB reading software / devices allow for users to change font sizes. Also, there is significant variation in screen sizes. Using relative sizes means that spacing of elements (indents for example) will always appear the same.
I’ll admit using relative sizes is hard. In some of the examples for Formatting Tips I’ve used exact measurements. I apologize for this but this illustrates how difficult it can be to take into account all of the possible rendering issues that can arise from such a diverse reading ecosystem available today.
Be careful when using relative sizes. Each unit is unique. Using a % for the indent will be a % of the screen width. While using a % for a text size it will be relative to the text size of that element. An em is always going to be relative to the current font size. Be sure to check the layout with various text sizes and screen dimensions. You can simulate (it’s not perfect) by opening the book in something like Sigil or calibre’s ebook-viewer and change the zoom and window size.