It's decently hidden, but absolutely worth changing in my opinion. If you open the pause menu, and click on "Settings" while holding CTRL+ALT, you'll have a new option at the bottom called "The rest".

In this menu, there's a field called "quick-bar-width". Changing this will indeed change your quickbar width! I've been using a 30-wide bar since the 2.1 experimental update, and it's been glorious. Anything else feels so... restrictive by comparison.