Here is my system. I brew 10 gallon batches on it outside and move it with a dolly. It's a bit maxed out but totally manageable. The boild kettle starts out full to an inch from the top. I figure if you are going to boil over it doesn't matter how much head space you have anyway. Normally I don't boil over but last sunday I did it 3 times. It was cold and I couldn't see through the steam to what was happening inside...
Pete