Answer: You can certainly trim it. There are two ways to approach this. One is to just cut it back quite hard all at one time in late winter or early spring. This can delay flowering while it regrows somewhat. Or, you can cut back a few branches at a time until eventually you have reduced the overall size. This way it can continue blooming without interruption.
The normal growth habit of tropical hibiscus is a bit gangly as you are seeing. Commercial growers often treat the plants so they stay compact and the chemicals wear off after a while. Pinching the new growth tips periodically can help it stay a bit more compact, but it will need routine pruning to stay manageable. Also, providing ample direct sunlight, especially in winter, can help. And, be sure you are not overfertilizing as this can cause legginess, too.
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