Once a subscriber unit served by a microcell initiates a call, it must remain in the coverage area of the microcell long enough to complete call set up and hand-off functions. This restricts the minimum size attainable by a microcell. This paper derives the relationship between the microcell size, the call processing time, and the probability that a subscriber unit, initiating a call, will remain inside the microcell coverage area for at least the duration of call processing. Under simple assumptions, time spent in the coverage area of the microcell before and after call initiation have the same cumulative density function, which is derived in this paper for traffic conditions encountered in highway and metropolitan settings.