Now there are tablets with SIM card slots and there are Phones which have a screens larger than 7 inch, so I guess the cellular radio or screen pixels density/size will not be a perfect solution.
To limit the application to smartphones only in Google Play Store you can use the "Device Catalog" and filter the devices that your app is compatible with.
In the device catalog, you can filter devices based on various criteria such as:
- Manufacturer
- Model
- Operating system version
- Screen size
- Density
- Hardware features (camera, GPS, etc)
By selecting the devices, your app will only be available to users on those devices.