Quite a dilemnia, you should have pruned it in the fall, so you're not cutting off the flowers now...but..long term goal should be a bushier healthier looking plant, not flowers.

Bareness is due to there being something lacking in the cultural environment. As far as "so big" is concerned these get a lot bigger than this, if you intend to keep it even smaller than it is now you are doing to be fighting the plant's genetic imperative the whole time.

If you search "hibiscus care" on the web various pages about growing tropical hibiscus plants come up. What they really want is to be outdoors in a suitable climate or at least in a warm, bright and humid greenhouse. Nevertheless the municipal library in my town used to have one of some size - as in maybe chest high - located right in the center of the main room that survived and even flowered, without being in close proximity to any windows.

@Embothrium, I bet they had a skylight over it?

or fluorescent tubes :)). libraries are usually kept very bright for reading.

I have a similar situation with mine: I am letting it grow a few more leaves on top to recover after winter. Then in a month or so i'll trim off 1/3 and put it outside to let it leaf out/branch out in better light.

perhaps the cuttings will root, though last year they failed...

I see lateral branches on yours fairly low on the stem: I would trim as low as that, but later as I said. or leave 1-2 nodes above the branching point.