The growing integration of renewable energy sources is driving microgrids (MGs) toward 100% inverter-based architectures, whose system stability and dynamic performance are tightly coupled with both the MG plant (i.e., resource type, number, and placement) and inverter control strategies. However, the existing MG design approaches typically overlook this coupling and fail to assess the dynamic performance during the design phase explicitly. Therefore, they often produce suboptimal configurations that do not satisfy the dynamic performance requirements once the design process is complete. To address this challenge, this paper introduces a plant-control co-design approach for 100% inverter-based MGs, which simultaneously optimizes the MG plant design and its control to minimize the system costs while ensuring the stable operation and enhanced dynamic performance. The proposed co-design approach systematically explores the feasible plant designs, evaluating their transient responses under disturbances using the optimal controllers. The MG plant design is formulated as a mixed-integer linear program, while MG dynamics are captured through nonlinear differential-algebraic equations. Electromagnetic transient simulation is used to validate the stability, compliance with IEEE 1547 standard, and dynamic performance, including power tracking in the grid-connected mode and disturbance rejection in the islanded mode. Case studies demonstrate the impacts of the number and placement of battery energy storage systems (BESSs) on the dynamic performance of 100% inverter-based MGs.