PVC coatted tarps 18-20oz
Size depends on you, I like to use the formula: lenght of trailer/2 + 3 feet (so it overlaps properly, some may say it's overkill but I like it to be neat...) has for the heights depends all on the carried equipement (some use higher for Canada hauling (for quad and bi-train) lower tarps for US (tandem trailer).
Plus there is some supplier that sell what they call steel tarp (very small but super strong) and wood tarps (high and medium strenght)
Price wise, I can't really say depends on your supplier. Could be from national retailer and the now newly infested Chinese maker market...
Hope it could help a bit. I do not have the science of it, so feel free to corrrect me if i'm wrong.
Cheers