I lived in Park Co for 25 years and was a Reserve Deputy there for almost 4 years. I was a JeffCo deputy for almost 10 years. There has NEVER been any sort of minimum lot size for shooting on your own property in Park or JeffCo since I've lived/worked there at least. .455's info is correct...at least for Jefferson and Park counties. Don't know about Boulder.
There may be HOA restrictions re: shooting on your property. But that's a civil issue between you and the HOA, not a criminal issue.
As far as I can remember there isn't a statute that specifically says it's legal to shoot on your own property. There are only those that state what is illegal...such as being under the influence, shooting recklessly, etc...