I'm moving a deck footing for an existing deck, the current one is in the wrong location and it's too small. What is the proper depth in Connecticut in order to ensure that it doesn't suffer from frost heaving?
You need to place the footings deeper than the frost line. Some research says the frost line in CT is around 42" so you'd do well to put the posts down 48".
The city will be able to verify this when you get your permit approved.
Count yourself lucky... In Ottawa, Ontario I had to put them down 60"... and they still heaved one year.