Either way can work. It's a lot easier to just tell them you like them, but if you can dig up the courage for a long speech go for it! The girl who I like -- I've wanted to tell her I like her for a while but I couldn't find the right moment. Recently though, the timing felt right and I just told her "I like you." She may or may not have gotten that I really like her, but I was finally able to say it at least.
So my advice is tell her as much as you can. The more the better (to an extent of course). I think just saying you like her is enough if the atmosphere is already good and it feels like the right time to say it. If you're doing it through a note or something, though, length is probably better.