Depending on the typeface, 10 point type is typical of most printed materials. 12 starts to look oversized in large blocks; my wife complains about my use of 12 point type on my resume—she thinks it's too big.
That's for typefaces intended to be used in body copy: Times, Helvetica, Caslon, Garamond, and the like. A stylized font may be unreadable even at 12. I'd say print out a paragraph in your chosen typeface and hold the page at arm's length. If you can read it easily, then it's fine. If you have to slow down, then it probably either needs to be larger or replaced entirely.