We derive simple engineering criteria which may be used as a guideline in MOSFET design. In particular, with respect to series resistance it is clear that a too high R S value is detrimental to device performance. On the other hand, pressing for a value which is lower than necessary may call for other unwelcome features (e.g. too deep junctions). Hence, a means for estimating the allowed R S values is of prime importance. We show how such criteria may be obtained and used for estimating the necessary quantities.