circular waveguide dimention
for pure dominant mode coverage of your 71 to 86GHz band with its (geometrical) center frequency of 78.14GHz I would simply select the following IEC waveguide size: "60153 IEC-C 799" with an ID of 2.67mm.
This waveguide is standardized in IEC 60153-4:1973 (2nd edition) as a so called intermediate size. Unfortunately in the standard currently in force, IEC 60153-4:2017 (3rd edition), they have "forgotten" to publish the table of intermediate sizes. However, the 4th edition of this standard - of which a draft version was completed only a few weeks ago - will bring back the intermediate sizes and also explain the different definitions for operating frequency bands.