I think my minimum "freezer is echoing like the Grand Canyon" number would be about 800 lbs. I haven't ever pulled the trigger that early, though. I spread the love around when it comes to protein, so when that smallish steer is starting to look tasty, I'll load up on lamb or chickens to get by until he catches up.
It is a highly personal question. We are feeding a family of four, so the cuts can't be too tiny. Give us a couple of years after the kids move out, and we may be more apt to eat a little fella.