as far as length, it depends on who you are giving the cd to. If you are giving the cd to someone who has a lot of other cds to listen to, IE a record exec or a known rapper or someone, then your tracks should be short and should get going pretty quickly. If you are giving your beats to a local or unknown mcee, then i like to use my full intros and let the tracks get longer. i always use a lot of vocal tags though.
I think a lot of the feeling and emotion of beats, especially of mine, comes from the intro and the build, so i like to able to display that where it is reasonable.