For whatever reason my bernicker seems to want to grow along the ground. It's a pain. Because of this I have cut it back every year trying to build branch strength and structure. I have also moved it after a year in the ground and I have probably had it for 2.5 years. I have knocked the fruit off but it has flowered every year. Based on this I think you could probably keep it very small as long as it is big enough to bear fruit.
Keep in mind I haven't fruited it at this size, this is based just on what I have seen.