I'm not familiar with UL testing requirements, but one way to get a range of impeller sizes approved would be to start testing with the maximum impeller size the casing will accept, and then remove the impeller and trim it to the smallest acceptable diameter for that casing size and repeat the testing. I would think that having a impeller in the middle of the size range, and then applying the affinity laws to determine an acceptable range for certification would also be acceptable, but that would need to be confirmed with the testing authority.