With what is described in this question, we would recommend a 2" length screw. What is most important when determining the length of a screw needed for a project, such as this, it is knowing how far it is from the top of singles to the plywood below. For a secure application, you will want to select a screw length that will go through the shingles and through whatever is underneath the shingles and into the plywood below. If a 1 1/2" screw will sufficiently penetrate the plywood, then use it.