Rose of Sharon is Hibiscus syriacus. Virtually all ROS will grow to approximately the same size, which is about 8-10'. I have never seen a ROS much exceed that height but I guess it may be possible. The training habit is not going to significantly alter the maximum size of as plant, which is determined genetically. You might get some additional extension because you are focusing the apical growth but I have no idea how much. I seriously doubt as much as another 5 feet! Grafting, also very common on standards or tree-formed shrubs, can alter or add to mature size significantly, depending on understock.

ROS can be invasive in various parts of the US. Look for the triploids named after the Greek goddesses - 'Aphrodite', 'Helene', 'Minerva' and 'Diana'. They are, for the most part, sterile.

ROS can be rather easily trained into tree form but this is usually done while the plant is still small and is accomplished gradually over a period of years.

Here is a link that might be useful: tree form ROS

there is one in my area that is just about the size you mention. i took the pictures this past weekend and, as i stood there, guessed that it was about twice my height, which would be 12'-15' tall, and a little wider than that. the trunk near the ground was at least 6" dia. it has been there as long as i have lived in this neighborhood (about 30 years) but i am sure it can't be as old as the home where it is found (antebellum). :-)
sorry about the picture quality. it was very windy; probably the early effects of tropical storm debby.
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I have 2 that were in the yard when we bought the house 11 years ago so I don't know the variety. Late fall after they loose their leaves, I trim each branch back to one or two nodes. They look great all winter, very sculptural. By 4th of July they are blooming and about 9'-11' tall. The fence in the background is 6'. They are messy, spent blooms like confetti below and loads of seedlings but very easy to pull.
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Jeff, that's the largest ROS I've ever seen! They don't get that large where I am, in Houston.

Something must be in my soil here in New York. I have two ROS trees (on the right side of the picture) that are taller than my cap cod style house. Those two are so big that they don't even bloom till August. I have 7ft one out back that is already in full bloom.