It depends on how fast you want to make your quilt. If you want fewer blocks that make up fast, then use a larger center and wider strips. If you want a bunch of smaller units, then use smaller/narrower elements.
This is where it pays to pull out your graph paper and colored pencils, and play with the finished look before you cut anything.
There are quilt programs out there that will do the same thing, but I guess I'm "old school."