If I were you I'd make 4 gallons )or even 5 if you want to use a blowoff tube) - more beer! And just dilute the 16qt wort with boiled and cooled water in the fermenter until you get your desired volume.
I just cracked a bottle of extract only hef with weihenstephan liquid yeast, but it's not carbed properly... and this is the second growler I've tested. They say to drink it young, so it's only been 2-2.5 weeks, but that should be enough time, no? This is the first beer I've made which hasn't carbed so am wondering what I did wrong. There were some bubbles but they were short lived and within 20 seconds it was flat, priming sugar level definitely wasn't a factor. Any advice would be much appreciated!