Help with Avidemux settings

[johnMk]

I have an MP4 file that was captured from a family VHS by someone else. The quality is acceptable, but the file size is quite large at 15 GB for a 2.5-hour video.

Dealing with the sound in Audacity has been no issue, and I have a pretty clean track ready to use. However, I've been trying to reduce the file size with Avidemux but haven't had any success.

So, what settings should I use in Avidemux to achieve a quality result similar to the original file while reducing the size to something acceptable for a video of that length?

Thanks in advance!

[Geo_log]

I've been trying to reduce the file size with Avidemux but haven't had any success

What exactly did you try?

[Urik]

Considering you say this is from a VHS footage, I assume the resolution is somewhere in the vicinity of 360p-480p-540p (SD). Unless it's been captured with some smart upscale to 720p-1080p (HD).
The size/length you mentioned lands somewhere at ~14 Mbps which is indeed excessive for SD resolution (and very good for HD).

I dunno who the file is targeted for (a family viewing on like a smart tv/nas or purely preservation), but
if you were going for compression efficiency v quality, I'd definitely consider h265 (HEVC). Either with x265 (software encoder), or Nvidia NVENC (HEVC). The former uses CPU, the latter needs an nvidia gpu (the newer the better).
It's way more efficient than h264 in spatial & temporal compression, i.e. preserving more details at comparable bitrate vs h264, or having similar quality with a lower bitrate. And almost any modern device that came out in last 5+ years should play it fine. Although of course globally h264 is still more compatible and supported, but is older and less efficient.

In regards to x265, there's a lot of settings, you will probably find plenty of guides/posts online about it (you can look up handbrake guides too cuz the settings are very similar).
The default CRF (Constant Rate Factor) setting of 20 is good start to see if you like the result, and you can use two-pass encoding which will take more time but in theory yield better results compression and quality wise.
Or go with average bitrate, I dunno, maybe between 1-3 Mbps? That should be plenty if it is not HD.

I'm not really a connoisseur of fine-tuned encoding parameters, but these just basic starting points.