I saw a recent official wix tutorial on youtbe a couple of days ago, and he was able to change the stack width through activated advanced options of h and w and then select fit to content, while me it says not possible as shown on photo so i don’t understand:
Can you share a link to the video that you watched - it’s hard to tell what might be happening without it 
In general, Horizontal stacks don’t support width measurements, since they will always hug the width of the content inside - whereas vertical stacks do support width 