I would like to keep it about 5-6 ft tall.
Since these flower, almost, constantly you can prune these at just about any time. You can prune to your preferred height with no problem. Just make sure to use clean shears.
This article will give you more information: https://www.gardeningknowhow.com/ornamental/shrubs/marmalade-bush/marmalade-bush-information.htm