Generally, use the Height attribute in the selected codec in TVH under Profile Settings and then set a bitrate under the codec settings. Alternatively you can set CRF (Constant Rate Factor) instead of bitrate.
There are many more things you can configure including hardware acceleration but to my experience Hardware Acceleration has been very problematic in TVH. Instead you can use your own ffmpeg binary and enable all kinds of acceleration there (QSV,VAAPI,NVENC) and then use this binary in TVH and pass parameters to it.
Check ffmpegs website for more details and info