Hello! I hope the response to your email question was helpful.
Regarding this question, I assume you are asking how to change the size of the video source of the game. In that case, as shown in the attached video I sent for help, when I right-clicked on the main screen inside TikTok Live Studio, you will see "Full screen", and when you hover your mouse over it, you will see "Fit to screen". If you require additional assistance, feel free to reach out to me via email.