circular wave guide design?

if i want to design CWG, that to design LNA at 11.7 GHZ,its dimensions so large in comparison with its similar at market , what i do?

Since there is no recognised standard dimension, CWG may be arbitrarly choosen into a limitated range of diameters.
Given a frequency range, CWG has more possible modes than RWG.
Given Fco=1, the TM01 is 1.3 Fco, TM12=1.66Fco, TM11=2.08Fco
TM11 is the most dangereous higher mode.

The best range of diameter should be choosen accordly to pattern requirements and that 1.3Fco<Freq<2.08Fco

Why CWG? Are you trying to copy a satellite LNB receiver design? You should realize that those are CWG because the satellite transmission is cirularly polarized. Those round feeds are not simple CWG, but complex structures that received either Right Hand or Left Hand circular polarized signals.

In general they have a circular horn, a mode converter, and rectangular waveguide to microstrip signal ports.