YES size does matter. One of my biggest pet peeves is that the manual that comes with machines don't tell you which one to use. I have scoured my manuals and the only info is a part number, thier part number. Why can't they just say class 15, L or N. Yesterday for the first time I saw a sewing machine box that specifies class 15. I'm going to email the company and thank them. Being consumer friendly should always be encouraged.