Does this instrument measure the dust in a factory lets say ?

yes, this would measure dust in a factory. I have one of these in my living room. If I close all doors and windows and turn on my air purifier then I will get a reading of about 100 on this thing. if I open my doors and windows it goes up to around 800 - 1,200. If I sweep the floor it goes up to over 5,000. If I burn something on the stove it is smokey then it go over 10,000. I would imagine that a factory with sawdust blowing around would probably be pretty high.

What is normal for small and large particles???

You could go to the EPA web site and look up "particulate matter". It is a very complicated subject, and even a little learning is helpful. D.M's answer is good. I wrote to Dylos and they responded that the words ("good, fair", etc. corresponded to US government standards for the important small particles (PM 2.5 and under). Those are important for people who are constantly exposed to small particle pollution, such as nearby freeways, etc. I have lost half my lung capacity due to interstitial lung disease and I keep the left count (larger one) under 300 in the 2 rooms where I spend most of my time.

I live by the side of a major interstate highway. Is this - or any - monitor able to let me know if auto combustion products affect my indoor air?

I can tell you if your PM counts are elevated but will not be able to isolate specific gases., and other microscopic particles.

Will this pick up drywall dust counts?

I have the Dylos DC1100 standard (not the Pro model) . I have been installing drywall panels and doing some AC outlet cutouts with an electric drywall cutout tool. When doing my cutouts , I have tested the air particle count with my Dylos DC1100 and the count values have increased by a large factor (10x to 100x ) compared to the values before using the cutout tool . My unit will pick up easily the drywall dust. I have also tried my DC1100 with a Shop Vac (with a normal air filter) running in my work shop and again the particle count has increased very much. It is very sensitive and easily picks up the count of these large dust particles.