circular waveguide TE11 mode

Hi,

What is the selection criteria behind the length of a circular waveguide, while designing for TE11 mode? Most design handbooks/calculators do not delve deep into the selectivity criterion for length of the circular waveguide.
Any rules of thumb would be helpful!

circular waveguide...as in a transmission line? I would keep it under a kilometer or two!

As a RESONATOR...i would not go more than 2 or 3 half wavelengths in length, since the resonance will get very narrow.

Length doesn't matter if you are going to use it as just a guide to your horn antenna etc.