How the plasma stability index beta and the fusion power density influence three performance parameters of fusion reactors burning deuterium-tritium and four advanced fusion fuel cycles was determined. The performance parameters include the total power produced per unit length of the reactor, the mass per unit length, and the specific mass in kilograms per kilowatt. The scaling of these parameters with beta and fusion power density was examined for a common set of conservative engineering assumptions on the allowable wall loading limits, the maximum magnetic field existing in the plasma, the average blanket mass density, etc. It was found that one should employ an entirely different strategy for the design of an engineering test reactor (ETR), designed to test components under high wall loadings and neutron fluences, than one would employ in designing a power plant reactor intended to produce the cheapest possible thermal power. An ETR should not be merely a scaled-down power plant reactor, but should operate at substantially different values of beta and plasma power density, and in some circumstances even use a different confinement concept and fusion fuel cycle.