You will need to install a user script manager extension to install this script.
Toggle between 16:9, 4:3, or default.
Note: this is intended for the previous/faster/better version of YouTube, but for still works in the current one. To get the better version back, simply open your console (
Ctrl + Shift + J) while on youtube.com and paste this in:
document.cookie = document.cookie.split(' ').filter(o=>o.indexOf('PREF=')!==-1).replace(';','')+'&f6=8;domain=.youtube.com;path=/';