How to make a 1920x1080 mp4 smaller in file size

[Digmen1]

Hello,
I have quite a few mp4 videos on a hard drive that are 1920x1080 in good quality.
I need to save some space
So what would I do to reduce the file size, but still keep good quality.
Do I reduce the bit rate or the resolution down to say 1280x720.
if I reduce the resolution, do I need to keep the bit rate the same as the original to keep good quality
I have XmediaRecode and Handbrake.

[Sharc]

To reduce the file size you have to reduce the bitrate. Filesize = bitrate x duration of the video. Reducing the resolution and keeping the bitrate will therefore not reduce the file size.
However, you could reduce the resolution which would allow to reduce the bitrate even more. Try and trust your eyes what is acceptable for you.
I assume that your video is progressive, so reducing the resolution to 1280x720 should cause no issues. If however your video is interlaced you need to deinterlace or bob it before resizing, otherwise the result will be messy.
When using Handbrake I would also tick the CFR option (force Constant Frame Rate output).

[hello_hello]

I'd resize to 720p. If you use x264's quality based encoding method (CRF) the final bitrate and file size will vary quite a bit (according to how hard the video is to compress). If you use 2 pass encoding you can pick the bitrate/file size, but the quality will vary.

I'd pick a CRF value that gives you the quality you're after and let the file sizes vary. CRF 18 is roughly "transparent". Lower CRF values increase the quality. Higher CRF values reduce it, and they require less bits. CRF 16 - 23 is a sane range.

The rule of thumb when encoding with x264 is to pick a quality and use the slowest speed preset you can stand. It also pays to enable the "Film" tuning.
I mostly use CRF 18, Tune Film, Preset Slow (or Slower) for the x264 encoder myself. I wouldn't use the x265 encoder. It's very slow and for 720p it probably won't compress more.

Handbrake encodes with x264 or x265.

The original bitrate isn't relevant. The video is decoded and the uncompressed version is re-encoded at whatever quality/bitrate you choose.

[Cornucopia]

If your playback systems can all support it, you could also use a more efficient codec (e.g. h265) which would give you equivalent quality at a lower bitrate.

Scott