the drop down list, also called auto completion, wont take up any disk space, unless you have tens of thousands or hundreds of thousands of those drop down words. it is simple to empty. I am guessing you are running IE.
at the top of the IE window, look for the MENU items, find the one named TOOLS, then go to Internet Options then Content then AutoComplete and click the Clear Forms.
you might want to also clear the History as it also fills up with user typed addresses, to do this, go to TOOLS, Internet Options then Clear History, while there you might also wish to clear the Temporary Internet Files, as that DOES take up disk space, if the settings are at the default level, you might also want to adjust it. cause it will take up alot of space.
TOOLS, then go to Internet Options, then Delete Files, then after deleting them, click the Settings in the same are as the Delete Files, adjust the slider til you have between 20 - 40 megs of disk space used for Temporary Internet Files.