So, some people have a vision of a fast, face eating flying nanite cloud. This wouldn't actually work (in a hard sci-fi sense). No matter your size, you're going to be operating in a speed of "body lengths per second." Basically, the smaller you are the lower your top speed. The top performers tend to be around 30-40 with endurance, with sprinters capable of up to 60-125 body lengths per second (sailfish, tiger beetle, cheetah) So a swarm of micro-machines 10 microns in length is going to be moving along at say .4mm per second (40 body lengths per second). That's 1.44 meters per hour. Not enough to fight even the lightest breeze or air convection. Basically, if a nanite cloud wants to move fast in a "hard sci-fi" manner, it needs to form up into an animal sized creature/construct first. EDIT: Fixed meters per hour. Goofed the calc.
—"I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve."- Isoroku Yamamoto