It could last them awhile if you start them smaller. I've found that they go threw a growth spurt between 6 and 20" then tend to slow down. If there weren't a lot of other fish a channel could probably live for some time in a 63x69. They are very hardy as well, and could stand the 80 degree summer temp if the pond is deep enough and reaches that temp gradually.
If you wanted a fish in there for life though, I would go with a bullhead. You could have more than one bullhead in there too, but I would only recommend one channel