How big is it now? Are you hoping to make it into a bush? There are various sizes of rowan depending on the variety but trying to keep a big tree small is very difficult. If you keep cutting it back you are unlikely ever to get flowers. Did the label say which variety you have or what its eventual height will be? Sorry this isn't quite what you were asking.

They don't tend to flower until they are about 8ft tall in my experience. I have a 12ft one that isn't getting taller but it is getting more branches and more flowers as a result.