[html] HTML: Select multiple as dropdown

Because you're using multiple. Despite it still technically being a dropdown, it doesn't look or act like a standard dropdown. Rather, it populates a list box and lets them select multiple options.

Size determines how many options appear before they have to click down or up to see the other options.

I have a feeling what you want to achieve is only going to be possible with a JavaScript plugin.

Some examples:

jQuery multiselect drop down menu

http://labs.abeautifulsite.net/archived/jquery-multiSelect/demo/