I have the dimensions of my project so I go and cut them the same length. You can do the math and figure out how many strips you need, I am not at all that organized, My finished project will be about 30"x 55" so I wanted them the short direction so I cut them at the 30" most people will want the strips to run vertical, you can make the decision when doing it.
Cut the same out of both strip sets, so I have one pile of flannelette strips and one denim set.