Ronald, I like doing both, acreage and a setback. On my end by the Benton line we've got a bit of everything, a couple of newer subdivisions and then somebody's 8 acres with a barn right behind them. It mostly works because people just talk to each other. A flat rule might break that.
On enforcement, Matthew's right that nobody's going out with a tape measure. I'd rather they go complaint-driven, same as most things. Somebody complains, then they measure. Otherwise it's a rule nobody checks until a neighbor is already mad.
The other thing I haven't seen mentioned is grandfathering. If somebody's had four hens in the back yard for ten years and nobody's ever complained, I don't think they should get a letter because the city picked a number. Write the rule for new setups and deal with problem ones as they come up.
As a gardener I'll say hens are good for a yard. Mine would be if I had them. They go after the bugs, and the manure makes great compost once it's aged. Goats are a different story. They'll eat a garden down to the dirt and walk right through a flimsy fence to do it. So I'd tie goats to real acreage and proper fencing, and keep hens easy.
Since the rest of the animal control stuff is still on the table, I'd rather people who actually have animals show up at the next workshop than just talk about it here. The city mostly hears from whoever shows up.