I took a look at your site…
The category select is full width do to a CSS declaration in the WP Job Manager plugin. It has this:
Unfortunately with the way it is defined it will affect all plugins which use the Chosen library to enhance select drop downs. The author should make a more defined selection so it only affects their plugin.
Even more unfortunate, since the !important flag was used there is no way to fix it from my side. You’ll have to contact the WP Job Manager dev to get this fixed. Apologies! I wish I could do more.