I've used a 7.5 Gallon kettle, you do have to keep an eye on it. Of course I've seen someone mention successfully boiling over a 15.5 Gallon keggle with just a 5 gallon batch so....
I added my first runnings, turned on the burner to get a head start on the boil, then, while I had my back turned (adding sparge water to the MLT), It boiled over bad enough to put the flame out. It probably had less than three gallons in a 15 gallon kettle.
I've been using an 8 gallon for awhile and it works, and I never used fermcap, just a watchful eye and heat management. It could boil over very easily.
I just upgraded to a 15 gallon pot, and it cost about the same as my 8 gallon (both Aluminum). $67 delivered. Huge and solid as a rock.
Fermcap and Defoamer 105 are silicone based liquids that are added to the wort while it is heating up. The active ingredients get inside the bubbles and destabilize them causing them to collapse.
Fermcap and Defoamer 105 are insoluble. They will stay on top of the wort and not be transferred into the fermenter.
It's a substance that disrupts surface tension in liquid and will almost eliminate foaming/boil overs. I use it in my yeast starters and it works well.