Stateless model checking (SMC) is crucial for productivity in concurrent programming, and its recent developments for C/C++ and weak memory models are remarkable. The state-of-the-art SMC for C, GenMC, efficiently verifies C programs based on C11 atomics and Pthreads. However, it does not support mixed-size access, which is access to the same memory region with different-sized types and ubiquitous, particularly in memory management. As a result, GenMC does not work for C/C++ programs containing memory management. To resolve this problem, we develop a method of adapting GenMC to mixed-size accesses preserving its optimality. We experimentally evaluate the efficiency of our extended implementation of GenMC and its efficacy for memory management programs.