It is set to Automatic.
No, no intention to publish it on YouTube. It will only be stored on an external HDD to be shown on a TV, but, nevertheless, I am wondering if it is possible to lower the file size without compromising video quality.
Moving to h265(which can still have an mp4 contrainer) will give you better compression but take longer to export. How long is your video 53Gb sounds massive.
Not sure what you mean with sounds massive, but I can tell you that the voice is for around 30 min. part of the video. For the rest no voice/sound. But I plan to add background music for around an hour.