This thread showcases some examples of ideal lips.
Ideal lips are as wide as the IPD, full and are symmetrical.
The oral commissures are upturned.
Apart from that ideal lips have a reddish color.
Matt LeMond
Ideal pretty much.
Richard Ramirez
Also ideal. Could be slightly more reddish.

Matt Lemond
Again ideal.
Jordan Barrett
Lacks a bit of volume, but also close to ideal.
Francisco Lachowski
Worst example of the thread. No clear vermilion border and asymmetrical lips.
David Gandy
Ideal lips.