Red twig dogwood can be a larger growing shrub, depending on the cultivar. Once it is dormant this fall or early next spring, you can cut out a percentage of the larger older limbs at the soil line. you can then prune back the remaining branches with hand pruners to an appropriate smaller side branch or bud. this is called renewal pruning. by removing a few of the oldest branches every two or three years, you will naturally keep the plant smaller.