If you go to the Lockscreen settings, there should be a "Clock Widget Options" setting where you can change the clock size, but that's about as much customization as you'll get there. However, if you go to set a Personal Message, you can edit the font, size, and color of both the message AND the time/date stamp.
If you'd like more customization options, you can always use a lockscreen replacement such as WidgetLocker (you basically turn your lockscreen into a homescreen and you can place any widget there) or the uberpopular DashClock.