Overflow box size??

[kuatto]

How big does the overflow have to be for my 54gal?I have a Aqua Tech 30-60 hob filter,is it of any use to me as an overflow(maybe just the filter box).
Any help I can get in planning this would be greatly appreciated
The tank is not drilled(already set up)so I'll be using return lines with elbows.I really have no idea when it comes to plumbing and set up of a sump.

Jim