Improving TCP performance has long been the focus of many research efforts in mobile ad hoc networks (MANET). In this paper, we address one aspect of this endeavor: how to properly set TCP's congestion window limit (CWL) to achieve optimal performance. Past research has shown that using a small CWL improves TCP performance in certain scenarios [M. Gerla et al., Feb. 1999], [Z. Fu et al., Apr. 2003], however, no comprehensive study has been given. To this end, we turn the problem of setting TCP's optimal CWL into identifying the bandwidth-delay product (BDP) of a path in MANET. We first show and prove that, independent of the MAC layer protocol being used, the BDP of a path in MANET cannot exceed the round-trip hop-count (RTHC) of the path. We further refine this upper bound based on the IEEE 802.11 MAC layer protocol, and show that in a chain topology, a tighter upper bound exists, which is approximately 1/5 of the RTHC of the path. Based on this tighter bound, we propose an adaptive CWL setting strategy to dynamically adjust TCP's CWL according to the current RTHC of its path. Using ns-2 simulations, we show that our simple strategy improves TCP performance by 8% to 16% in a dynamic MANET environment.