State-of-the-art model checkers employing dynamic partial order reduction (DPOR) can verify concurrent programs under a wide range of memory models such as sequential consistency (SC), total store order (TSO), release-acquire (RA), and the repaired C11 memory model (RC11) in an optimal and memory-efficient fashion. Unfortunately, these DPOR techniques cannot be applied in an optimal fashion to programs with mixed-sized accesses (MSA), where atomic instructions access different (sets of) bytes belonging to the same word. Such patterns naturally arise in real life code with C/C++ union types, and are even used in a concurrent setting. In this paper, we introduce Mixer, an optimal DPOR algorithm for MSA programs that allows (multi-byte) reads to be revisited by multiple writes together. We have implemented Mixer in the GenMC model checker, enabling (for the first time) the automatic verification of C/C++ code with mixed-size accesses. Our results also extend to the more general case of transactional programs provided that the set of read accesses performed by a transaction can be dynamically overapproximated at the beginning of the transaction.