A two-fluid theoretical model of heat transport in tokamaks is developed based on microinstabilities. A τE scaling of the form has been determined from this transport model. The theoretical τE scaling is used to determine the smallest size tokamak fusion reactor for two cases, ignition and Q = 5 steady state. Parameters are in good agreement with Tokamak Ignition/Burn Experimental Reactor II designs based on empirical Kaye-Goldston τE scaling.