Normal books usually have a font size of around 10 pt., depending on the font.
The measurement of fonts is actually kinda strange. They're measured in points, but it's not the size of the letter that is measured (think about how different fonts are radically different in size, even at the same point size--play with it in your word processing program if you don't believe me).
To understand it, you have to think back to when printing was done with moveable type. Each letter would be shaped on the end of a small piece of metal. Most of the time, there's significant space between the outside edge of the metal and the form of the letter. The needed letters would be lined up to form the lines of whatever was being printed, and they had to line up perfectly. So the size of the metal blocks (I'm sorry I can't remember the name for them) had to be the same from top to bottom. So that's what was measured--they height of the piece of type, or of the row created by lining up all those letters.
Even today, a font's point size is the size of the metal type that font's set of letters, numbers, etc. would fit on (or did fit on, back in the day, for some of the older fonts). Each font might allow for more or less space around the edges of the type.
In book publishing, the spacing between the lines is also described in points, rather than "single spaced" or "double spaced" or whatever. So a typesetter or compositor or book designer will indicate that the text is set "11/13" or "11 on 13"--meaning, 11-point text with 13 points of spacing. Or whatever the designer chooses. More than you wanted to know, yeah?