I'm not very sure about your questions but samples can be any lenght you want them to be depending on the application. My samples range from one note to entire phrases. I sample what I want and then I stop it. Most of the time I don't need to cut it at all. The few times that I do cut a sample is when I decide not to go with my original idea and use only a piece of the sample. I will sample by both of the examples you used...(bar lenght and time). It all depends on what you want. If they don't flow correctly as in a phrase sample...maybe you need to adjust the pitch(speed) or time strecth it so that it matches whatever you're putting it with. Hope that helped you.
--dac