This paper proposes a data-unit-size distribution model to represent the retransmitted packet size preservation (RPSP) property in a scenario where independently lost packets are retransmitted by a stop-and-wait protocol. RPSP means that retransmitted packets with the same sequence number are equal in size to the packet of the original transmission, which is identical to the packet generated from a message through the segmentation function, namely, generated packet. Furthermore, we derive goodput formula using an approach to derive the data-unit-size distribution. We investigate the effect of RPSP on frame size distributions and goodput in a simple case when no collision happens over the bit-error prone wireless network equipped with IEEE 802.11 Distributed Coordination Function, which is a typical example of the stop-and-wait protocol. Numerical results show that the effect gets stronger as bit error rate increases and the maximum size of the generated packets is larger than the mean size for large enough packet retry limits because longer packets will be repeatedly corrupted and retransmitted more times as a result of RPSP.