[Coller] Altering the slider size

I was wondering how I could reduce the slider size on my website to at least half of what it currently is so that I could move the sidebar right to the top and be able to match the resolution of my pictures. (https://www.theunderwaterpostoffice.com/)

Also, when I hover on a tab that I already have open, the screen shifts a few pixels upwards but I can’t work out why.

Cheers!