How big are little boys?

So, how big do little boys need to be to use the boys bathroom by themselves? I'm now on my second son and still don't have a good answer. I so appreciate the businesses who have installed family restrooms. This is wonderful. But, many places still only have men and women restrooms. Enoch refuses to use the 'girl bathroom'. When the restroom consists of a single toilet and a locking door, I'm fine letting Enoch go in by himself. Of course, I stand guard outside and I don't care what kind of dirty or strange looks I get. But, when it is stalls and I can't control who goes in and out, what do you do? Let me know. Also, check out my poll and leave your opinion.
Two funny bathroom stories (just so we can remember then and torment Enoch with them later on):
1. At Arctic Circle, Enoch refused to wash his hands because the liquid soap was pink and that is girl soap. We had to use hand sanitizer in the car.
2. At the library today, Enoch was freaked out because the toilet water was blue (from cleaner). After a few minutes, I could hear him behind the door laughing hysterically. He kept yelling, "It's green! It's green!"