This paper evaluates the impact of the operating modes, power consumption, and placement of temperature sensors against the heater to the design of a calorimetric flow sensor, for the range of 4.7 to 56.5 liters per minute (slm). In contrast to previous works most of which simply indicated the choice of various design parameters rather than providing a justification, this work provides useful guidelines for optimizing low-power small-area flow sensors for respiratory monitoring applications. A figure of merit (FoM) which is defined as the product of power consumption and sensor size, the two most challenging design parameters in developing small medical devices and systems, is proposed for quantifying flow sensor performance. Although the analysis and simulation was drawn upon designs in the mm scale, a similar optimization process can be applied to flow sensors of any size.