Some varieties of cosmos are taller than others....but pinching helps.
Pinch once when they are about 6 to 8 inches tall then pinch the new sprouts when they are about 4 inches. You will delay blooming, but they will be shorter.
Try the Sensation cultivars.
Linda C

To help avoid powdery mildew, make sure your plants aren't tightly packed together so there's some air circulation. Water at ground level and don't get the leaves wet.

I would agree with the answers above.

I have a loose soil that doesn't support tall plants when it's wet. A couple of years back a small bed of cosmos were blown down flat to the ground. The stalks didn't break but there was some root disturbance. The side branches then each developed like upright plants and bloomed at a much reduced height. A disaster that turned out to be a good thing.

I have thought of growing some more and either do them as larger transplants planted in a laid-over fashion, or grow in rows and then pry them over with a shovel.

{{gwi:7741}}

Verbena bonariensis is a tall, sturdy see-thru plant with bluish/lavender blossoms. Mixed among cosmos that otherwise flop it will hold up the cosmos and prove to be a winning combo. Both are very drought tolerant.

The tall verbena self seeds generously (as does the cosmos) so you will have more plants than you want the following year, but spend a little time thinning and you will have a perpetual annual bed. Wish I had a picture to share but this was before my digital camera.

crude but effective. we know exactly what you are getting at.