The crux of the Delay-Tolerant Network (DTN) routing architecture lies in the physical transportation of messages by the resource-bound nodes. This requires a demanding (node) buffer space for a seamless functioning of the routing protocols. An analytical instrument to systematically quantify this buffer space is vital to wield the buffer loss. Buffer dimensioning is therefore essential to design a practical and efficient DTN. In this letter, we address the problem of quantifying the buffer size of DTN source nodes, under replication based routing protocol, using Markov modulated processes and large deviation techniques. Our proposed dimensioned-buffer model is shown to exhibit a routing performance of an equivalent infinite buffer model. Furthermore, our analysis is validated via extensive simulation results. © 1997-2012 IEEE.