In this paper, size based adaptive backoff algorithm (SBA backoff algorithm) for IEEE 802.11 DCF has been developed to provide quality of service (QoS) requirements. SBA backoff algorithm assigns different priority levels based on packet size. Also, SBA backoff algorithm was utilized in the mobile ad-hoc network (MANET), which is a collection of mobile independent nodes. Because of dynamic topology, IEEE 802.11 DCF backoff algorithm suffers from a lack of adequate performance in MANETs. QoS requirements such as throughput, end to end delay and jitter were enhanced by applying SBA backoff algorithm in the MAC layer.