I like to use a horizontal overflow, mine has 4 1" drains across the back and top of my tank. you can see it in my build thread,
http://www.talkingreef.com/forums/me...-progress.html
As for hole size a 1" bulkhead drains about 600
GPH if its at the bottom of the tank and about 400GPH if its at the top. You can figure out how much flow you need depending on return pump and head loss. My overflow box is made of glass but could easily be done with acrylic, black to hide the bulkheads. Just my two cents
