Stock tire sizes can be found in owner's car manual. There are also many other places the manufacturer may write the size of the truck tires, such as the driver's side door, gas tank latch and glove box.
Although it may be redundant, another place to look for the stock size of the tires would be on the existing tires themselves. Tires have their sizes written on their side walls.
To safely purchase a set of tires, a consumer should be able to properly read its specifications. The size uses two groups of characters to separate the width, height, rim diameter, traction and speed rating.