I added some things to my above comments. Reread it. Size of spillway will depend on watershed and if a horizontal outlet pipe is used. Anti-seep colars are a very good wise investment on outlet pipes especially if you want the water level to ever be above the exit depth of the pipe. Aquatic ecosystems sells anti-seep collars that fit various sized pipes. Two or three would provide good anti-seepage insurance.
http://search.aquaticeco.com/index?D=Ant...nti-seep+collarUsing betonite mixed in with clay packed around the exit pipe is good cheap, leakage insurance. If you don't use an outlet structure I would spend a little extra money to get an oversized better built spillway. For spillway design talk to someone knowledgable about the specifics for your pond and watershed. If it was me I would get two or threee reliable experienced opinions. Good or the best available technical advice should be similar in nature. Subjective opinions and guessing will vary in content.
Some of my information above came from the pond planning and building book - "Perfect Pond... Want One" by Bob Lusk.
http://www.pondboss.com/store.asp?c=8
