The operation of a tandem mirror reactor (TMR) at fractions of full power is an important part of the power plant operation. It has been shown that fractional power operation can best be achieved by controlling the plasma radius, leaving the rest of plasma parameters unchanged. The physics of magnetic confinement in a TMR can be used to design the desired radius control (RC) system. It is proposed to bias the field lines through the ends of a tandem mirror device and control the radial electric field in order to guard against rotational instabilities and ensure nonstochastic radial transport. In this scheme, plasma on the unbiased field lines suffers rapid transport and is lost. As such, the potential control of the direct converter plates as well as the location of the grounded halo dumps requires the flux tube containing the core plasma to match the direct converter area at all times. Based on this effect, an RC system can be devised that uses the conservation of the magnetic flux to adjust the field near the direct converter such that a flux tube with a smaller radius in the device matches the direct converter. Field lines outside this flux tube are not properly biased and cannot support core plasma. A practical design of such an RC system for the Mirror Advanced Reactor Study conceptual TMR is presented. The power consumption by the RC coil system is modest and no engineering difficulty in the design and construction of such a system is expected. Utilization of such an RC system in advanced tandem mirror experiments can also be of great use in the study of such various phenomena as radial transport, control of the radial electric field, halo plasma production, radio-frequency absorption, etc.