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The reference diameter for the ECH transmission lines is presently 63.5 mm. Analyses of the heat generation and removal in 63.5 mm corrugated waveguide components were reported [R.A. Olstad, et al., Proc. of IAEA TM on ECRH Physics and Technology for ITER, Kloster Seeon, Germany, 2003, http://ipp.mpg.de/tmseeon; R.A. Olstad et al., ''ECH MW-level CW Transmission Line Components Suitable for ITER,'' to be published in Fusion Eng. & Design (2005)]. Those analyses concluded that the temperature of all components could be kept to acceptable levels, even with operation at 2 MW cw per transmission line. Recently interest has been expressed in the community about the possible advantages of using a smaller diameter waveguide for ITER, particularly because of limited space available at both the equatorial and upper launchers. In addition to ameliorating the space constraints, there could be large cost savings for a modest diameter reduction in certain transmission line components, particularly gate valves and CVD diamond window assemblies at the entrance to the launchers. Results of a tradeoff study on ITER waveguide diameter are reported. Diameters considered range from 45 mm to 63.5 mm; consideration is also given to the possibility of tapering down to 31.75 mm at the launchers. The most critical issue for smaller diameter components is the increased losses and increased power densities. These lead to more demanding cooling provisions and higher operating temperatures for components such as miter bends, power monitor miter bends, bellows, dc breaks, waveguide switches, and waveguide sections adjacent to miter bends. In addition, the overall transmission efficiency of the ITER transmission lines would be reduced.