I would use a consistent size for the blocks, but then I am a traditionalist.
If you use the same size square in the center, the same width of strips, and a consistent seam allowance, your log cabin blocks will come out the same size and you can use different layouts when they are done. You won't have to trim or fudge to make them fit, and the pattern will be more even.
If you want to see what it looks like using different sizes, draw a few on craft paper and put them next to each other. That way you can make a better decision.