I have talked to Scott, the keeper of the filtration system, some years ago about this. he said they had a 3" line years ago but someone was bringing in a large tanker truck and filling it up, then selling the water to others. On top of that, there were a few times the water was left on. This put their test experiments in jeopardy. So they shut down the big hose and went to a garden hose connection.
I can only assume that when they tore down the building and put in the new one along with moving the hose to it's current location, they left it as a garden hose for the same reasons.
As far as how much can you take, take what you need for your personal use. Don't abuse the system or we may all loose access to it. I have a 300g container that I fill up and have had no issues with them. But I have a 500 G system and use all of the water I get.
If your going to get a large amount like I do, be courteous to others. I've stopped my filling several times to let others get a quick 5 or 10 G of water.