Differentiated service network (DiffServ) provides an architecture is scalable and capable of differentiating applications' quality of service (QoS). However, if the flows use packets of different sizes, resource sharing is no longer fair. Now for some applications, such as Internet telephony, it is more natural to adjust the packet size. We study the impact of variations in packet size on DiffServ and propose methods to remove the throughput bias resulting from the use of different packet sizes. The paper suggests a solution that involves an resource discovery (RD) feedback loop and fair intelligent admission control (FIAC) scheme for each DiffServ domain (RD-FIAC-DiffServ). The scheme is characterized by three fundamental features. First, the RD loop introduces the virtual class-unit that is used to measure the available resources rather than packet size. Second, the RD loop infers the availability of network resources, yet for scalability, it does not involve core routers. Lastly, the admission control module understands incoming traffic's requirements, reconciles with available resources via the RD loop, and admits traffic intelligently according to the FIAC algorithm. We evaluate these designs through simulation and conclude with some concrete findings that RD-FIAC-DiffServ are robust enough independent of packet size and also can be used to improve fairness among flows within the class.