So you want to design a pyramidal Horn.
Start by defining what kind of horn. Frequency, bandwidth, feed etc. The desired electrical characteristics are important too. Yogi has a quote that went something like this: If you don't know where you are going you won't know when you get there. Figure out where you want to go. (It is your specification so feel free to modify it if you need.)
A good starting place for pyramidal horn design is Kraus. Some like Balanis as an alternate. Microwave Journal published a series of tables years ago that are useful as well.
Once you have the above worked out you are ready to tackle the simulation using one of the many tools available. CST is just one of the choices.
As a very general statement, pyramidal horn antenna are usually in the range of gains from around 10 dBi to about 20 to 25 dBi gain. Any higher and they get too big. Polarization is typically linear and bandwidth is that of a standard rectangular waveguide. E and H plane pattens are usually different with the E plane sidelobes generally elevated relative to the H plane. A good place to get an idea of horn performance and size is to review a set of "standard gain horns" offered commercially from a variety of sources.