Make choice box of quicksearch automaticaly resize


I would like to make the choice box of quicksearch resize to the biggest str. I dont really know how to describe it better, so i would like to know where i should modify the code. There is a screen shot to make u understand what i want. The first one is the normal one, the second one the one i want.
Its not only for that field, i want it to resize for all field that use quicksearch.

Thank you.