The font-size is defined in the import settings of the font you're using. See also: Importing True Type Font files (.ttf), so if you want different font-sizes, what you'd usually do is duplicate the font (select font, hit cmd+D) and import it with different font-sizes.
Regarding your other question: I'm not sure if selecting text works in GUI Text at all. It does work when using UnityGUI so unless you're on iPhone it may make sense to use that instead. On the iPhone, UnityGUI may drag down performance so in that case that may not be an option for you.
EDIT: On iPhone, it really depends: If this is in a menu (not in the main game), using UnityGUI should be just fine. However, you could also use Objective-C to do a Cocoa-based GUI.