What does one need to know about mesh settings in CST while designing any antenna?

As tht title says, I dont know, what is the mesh settings about and how to know what mesh size is right for my design
please help. thank you

Look into the help section of mesh settings. Generally, a more complicated structure requires mesh that is more dense. If you are simulating a simple structure of say, dipole antenna, the default settings would be good enough and in fact, you may use sub-gridding (under specials tab in mesh properties) to decrease simulation time.
It is a trade-off between accuracy and simulation time. A more dense mesh provides higher accuracy but also increases simulation time.
Hope that helps.