Final answer:
The throat side of a fillet weld gauge is used for measuring the size of a concave faced fillet weld because it assesses the theoretical throat distance.
Step-by-step explanation:
When measuring the size of a concave faced fillet weld, the correct side of the fillet weld gauge to use is the throat side. This side is used because it assesses the shortest distance from the root of the weld to its face, which is crucial for measuring the theoretical throat of a fillet weld, especially when it's concave. The options provided, such as the leg side, straight side, face side, and toe side, correspond to other aspects of weld measurements but are not the correct choice for measuring a concave fillet weld.