Info: The pond is .25 acres. Only filled by rain water and some slope from back yard. Rains alot in winter not much in summer, Im in south La.

Well i had just fininshed my pond and had a couple of lil rains maybe a foot or 2 of water in pond. I figured i still had time to put in an overflow pipe. Well we just had a huge rain storm and it overflowed my pond in to my back yard! I couldnt believe it. There is no levee around the pond in my back yard so that water could drain from the back yard into the pond. So my quesion is whats a good size for an overflow pipe? 4"-6"12"??? The other problem is it has to drain into my side ditches which is just a hair lower than the current water level.
Thanks!