$ScaleSize Does not work correctly about slider

[jssor]

The logic is correct.
As jssor slider will always keep aspect ratio, I noticed the height is still 400px after scale, some area bleed outside.
For your case, I think the $ScaleWidth is closer to your approach.

Btw, you can try the parameter maxBleeding (from 0% to 100%), it means how much allowed to bleed outside.
default value is 1 (100%).

jssor_slider_instance.$ScaleHeight(width, height, 0); //0% allowed to bleed outside, all keeps inside
jssor_slider_instance.$ScaleHeight(width, height, 1); //100% allowed to bleed outside, allows up to 100% to bleed outside