Custom CSS to resize elements in multiple categories

Well, I think you could use the CSS begins with selector to select all div’s that begin with category-

div[class*='category-'] #main { width: 300px!important; }

so as you add them, it will still affect any class that begins with them. Then, assuming you have 1 or 2 categories you don’t want to have the property, you could simply override them AFTER the rule above.

div.category-44 #main { /*do something else*/ }