Looking at a ruler where the finest graduation is 0.5 mm, I can clearly see the white space between the graduation marks. From a photo on Wikipedia, I see a Brevipalpus phoenicius that the scale shows to be about 0.25 mm long.
Brevipalpus - Wikipedia, the free encyclopedia
This 0.25 mm would be at the lower end of what I can discern. I wouldn't be able to count the legs on a Brevipalpus phoenicius without a hand lens. But, as with other predators searching for prey, at least I have noticed this to be true for cats, I watch not for the prey but for motion. If the smaller mites were motionless then it would be very hard to find them because they would look not much different from a speck of dust. So, my hunting technique is to work at night, after the lights are off; I will shine a flashlight on a plant and watch for vermin scurrying away from the light.