Perhaps others have other advice, but here's what I think. You could go about it two ways:
Wait until spring when the new growth comes out. Trim back some of the new stems at the ground level. That'll keep it in check.
OR:
I suppose you could dig it up and cut off some stems that have roots. This could work in theory. I'd still wait until Fall though. Trying to do it now, in summers heat, would really stress the plant out.
Barb

We have several very large, hardy hibiscus that we control by pruning. Cuttings that are at least as round as my thumb (I have a small thumb) are trimmed to about 14", stripped of all greenery and stuck into a 5 gallon nursery-type planter. I put just enough dirt to keep the stalks upright and keep it very damp in a shady part of the garden. I have several varieties so each planter contains all the cuttings for a single variety. This method works at any time of the year in our warm climate. I give away hibiscus plants to friends & family.

Hi, I divided my mature hibiscus this past spring with excellent results! I dug up the whole plant just after the first shoots were popping up and shoved a spade into the center of the root ball. Then divided each half a second time. Ended up with 4 much smaller hibiscus plants. This was an experiment I decided to try as I have 2 in a relatively cramped area and wanted to see how it would deal with the trauma of the division. I left the other alone for comparison. The hibiscus I divided is currently only 3 feet tall and has nice healthy stalks with buds. Just the size I wanted. The undivided hibiscus is over 5 feet tall and is wildly out of control. I plan on dividing it in early fall. I agree that you should wait until the temps die down a bit more. Wait until the average temps are in the low 70's to 60's. Good luck!
Tracy