Hey,
I have browsed through the configuration options, but I cannot seem to find the command for what I want. Basically, I want the mpv window to open by default at Half Size rather than Normal Size:

Is there a command I can input into the config file?
Add window-scale=0.5 to your config.
^ Yep, that seems to have worked. Thank you.
Most helpful comment
Add
window-scale=0.5to your config.