Dumb as a door post.
...Not the sharpest pencil in the box.
If wishes were horses, beggars would ride.
...and a miracle occurred... This was often used at work when we'd run into a problem on a project. It meant that somehow, someday, we would solve the problem, but not today.
From my father, who was from Massachusetts, but traveled considerably as a young man:
If the dog hadn't stopped to poop (different word actually used), it would have caught the rabbit.
"If" is the biggest word in the English language." Used when explaining that you 'almost' did something, and 'if" such and such hadn't happened, you would have done it. It also meant we were in trouble.
You make a good door, but a lousy window. Used to tell us that we were standing in front of the TV and blocking the view from the couch. I've never heard it used anywhere but in our house when we were kids. Of course, I have passed it on.
Your legs are younger than mine... Preceded being told to get something by one of our parents.
You'd argue with the devil himself! Usually my Mother said this...until the day I responded with "If it meant the difference between heaven and hell, yes, I would! Mom never used that one on me again.