The bond properties of an interface in composite structures are generally considered as local mechanical characteristics. Through experimental testing and analytical study, it is shown in this work that the interfacial bond characteristics are generally size dependent, which involve both Mode II and Mode III fractures. For fiber-reinforced polymer (FRP) externally bonded (EB) to concrete members, this size effect is called the width effect. Test results in this work show that there is a central region on EB FRP-to-concrete bond face where only Mode II fracture is involved, and hence, the bond properties are size independent. The measured bond test results in the central region are used to determine interfacial bond properties that are size independent. The global test results or overall bond responses are subsequently used to determine the width effect and its model, through rational reasoning and regression of test results. The proposed model includes new factor that has not been considered in the width effect modeling, and consequently shows a better performance.