Why do you want it to only run on non-tablets? Is there a feature (like SMS) you need? If so, add a uses-feature tag in your manifest. If its based on screen size, why not just scale up and get more users?
Also, screen size means nothing for phone vs tablet. My phone is a Note 2, it has more pixels than original generation tablets. You could disable all the devices you don't want on the play store, but it would be a pain.