In this paper we face the buffer dimensioning issue in which it is required to estimate the minimum buffer size of the packet/speech multiplexer needed to meet various traffic conditions with a specified quality-of-service (QoS) objectives and fixed source statistics. An important QoS measure that we study in this paper is the loss probability. The approach, based on the fluid flow analytical model, whose input consists of the superposition of homogeneous ON/OFF sources, is often used in literature for modelling packet/speech multiplexer: We also use this model. However, unlike the traditional fluid-flow approach based upon spectral expansion solution, we use matrix-analytic methods which give us closed-form exact equation to calculate the overflow probability. Further, by analyzing in asymptotic regime, we get an approximate equation from which, with simple algebraic manipulations, we get an equation for the minimum buffer size estimation.