Sizes range from 1/12 all the way up to 1/5 scale. The most popular are the 1/10-1/8 scalers. Racers off road are mostly 1/8 scale buggies and truggies. As far as how they work i dont know how to answer that. What do you mean by how do they work? Nitro or electric? Or what type of terrain they will run on? If your getting 1/8 scale then you will be ok to run on most all terrain except for very tall grass and rock crawling. For crawling you need a crawler. For racing on tracks and all out bashing you want a good 1/8 truggy for best but 1/8 buggies will do too. I myself want at least one of each but a truggy mostly for racing.
If your referring to how the scales work. Its just up to the manufacturer and how they wan to classify it. Traxxas for example calls almost all of their kits 1/10 but kits like the Summit, Revo, and Maxx's are all closer to what most companies label a 1/8 scale.