Set Resolution Limit to Custom -> define the limit you want.
Set Anamorphic None.

The scaled size will be calculated automatically.

Set Resolution Limit to Custom -> define the limit you want. Set Anamorphic None.

The scaled size will be calculated automatically.

I use that option to change the resolution of a single file and it kinda works, yes, but it doesn't work for 100 files for example. How can I set the largest side to 720p for example and batch through the folder? What if the amount of files in the folder is above 1000 and they all have different dimensions?

The dimensions of the source don't matter. Resolution Limit just clamps the resolution so it won't allow the resolution to go above either value.

I.e it automatically calculates the highest possible resolution, that maintains aspect whilst meeting both W & H limits. By default it won't upscale either.

You can also save this as a preset (and default that preset) once your happy with the results.

HandBrake does not have the specific concept of longest or shortest side. If you follow @sr55's advice:

Set Resolution Limit to Custom -> define the limit you want.
Set Anamorphic None.

You can set square dimensions using your desired value for longest side, and it should work.

So, try custom resolution limit 1280x1280 with anamorphic none and see if that does what you want.

HandBrake does not have the specific concept of longest or shortest side. If you follow @sr55's advice:

Set Resolution Limit to Custom -> define the limit you want.
Set Anamorphic None.

You can set square dimensions using your desired value for longest side, and it should work.

So, try custom resolution limit 1280x1280 with anamorphic none and see if that does what you want.

Thank you, I didn't think about limiting BOTH sides to 1280 with anamorphic disabled, my bad.
I tried something similar but do remember videos being stretched in Telegram but not in MPC (K-Lite). I will need some time to investigate if it works properly with every video that I have.