I've made a handful of log cabin quilts and I do believe I've used 2 1/2 in strips for most of them, but I did just make one with like 1 1/4 or 1 1/2 in strips (I can't quite remember) and it also looked great. It totally depends on what kind of look you're going for. I would draft out some blocks using different sized strips to see what you think would look nice for your project.