Final answer:
For measuring the size of a concave faced fillet weld, the throat side of a fillet weld gauge is used, which measures the shortest distance from the root of the weld to the weld face.
Step-by-step explanation:
To measure the size of a concave faced fillet weld, one would use the throat side of a fillet weld gauge. The throat side, sometimes also referred to as the concave side, is designed to measure the shortest distance from the root of the weld to the face of the weld, which is typically the theoretical throat of a concave or convex fillet weld. In the context of the provided reference information, TABLE 19.2, none of the options directly correlates to the parts of a fillet weld gauge, but for measuring weld sizes, it's important to understand that 'the length of a side' could refer to the leg of the fillet weld, which is not the correct answer in this case.