Accurately estimating the effort required for implementing change requests remains a critical challenge in software engineering, especially in microservice-based software architectures (MSSA). Traditional functional size measurement methods often fail to capture the distinct characteristics of MSSAs. To address this limitation, we propose a change size measurement method based on MicroM, a size measurement approach specifically developed for MSSAs. The proposed method counts added, deleted, and modified events across functional, architectural, and algorithmic levels, and includes the number of affected initial requirements. We conducted an exploratory case study with 18 change requests and built four regression-based effort estimation models. The results show that combining event counts with the number of affected requirements improves estimation accuracy. Our method provides a more precise and context-aware way to estimate change-related effort in MSSA projects.