As areal density in hard drives increases, the bit aspect ratio (BAR) becomes an increasingly critical design issue. In this paper, we use the detection signal-to-noise ratio (SNR) as a system-level indicator to establish the relationship between the optimal BAR and all major recording system parameters that relate to the head/medium combination and servo as well as the read channel. We address practical and fundamental issues including thermal stability, medium and additive system noise, and equalization and track misregistration. The results of our analysis generally agree with the recent consensus to move toward lower BARs but also caution that the optimal BAR is a strong function of the servo capability. The results of our BAR analysis also indicate that at high areal densities, the ratio of PW50 to the bit cell is considerably smaller than found in today's drives.