Magnetics are key components of conventional power converters, but they are often the bottleneck to achieving high power density due to their size, weight, and poor performance at small sizes. Piezoelectric devices, when operated in their inductive regime, can serve a purpose similar to that of magnetic components and offer favorable scaling properties as components are miniaturized. Several sources have demonstrated the viability of piezoelectric-based power converters, but selection of the optimal material and component size is limited by a lack of data on the performance of these materials at high drive levels. This work aims to fill that gap by collecting data to more completely characterize the losses in piezoelectric resonators owing to both mechanical and dielectric effects. To account for mechanical losses, the variation in resonator quality factor is examined across a range of drive levels for multiple resonator sizes, frequencies, and materials. By normalizing the collected data, material trends are derived that can predict mechanical losses under high drive levels, offering more insight into realistic converter operation than the currently available small-signal data sheet values. Additionally, a method for measuring high-power dielectric loss is presented, with results showing that the small-signal loss tangent provides a good approximation of losses even at higher drive levels. Based on these trends, implications for converter efficiency and selection of material and dimensions are discussed.