In this paper, the expressions for the forced termination probability, blocking probability and aggregate goodput of a single channel SU network with fixed packet size are derived. For a given SU data rate and fixed header length, it has been shown that there exists an optimal payload length for SUs packets which maximizes the aggregate goodput. Using the derived results, the impact on the aggregate goodput by three channel allocation methods i.e., intuitive, optimal, and proposed heuristic is investigated in a multiple channel CR network. Numerical studies show that the goodput of the proposed heuristic method is about 90-95% of that of the optimal method, and about 20-25% better than that of the intuitive method. Simulation results are also included.