Note that I have heavy duty allergies and don't use drier sheet for anything.
The size of your blocks in unimportant. It should be something you are comfortable with. I have done 18.5", which is too big to make the kind of free form pattern I enjoy. I have made 6" subblocks, which I posted, I usually now use either 9" or 12" (finished size) blocks. If you want to go to the work of serious layouts, you could have different sizes and shapes in a single quilt. I have not done that one ...YET.