14" will be fine. How tall are you making it? I have a mini-UDS with the basket about 8" in diameter. Filled to the top, adding lit coals ala The Minion Method, I can get burns of 8 hours+.
You're going to fill the basket with charcoal, lump or briquets, leave it unlit, and add 8-15 fully lit, glowing briquets to the top. This will give you a long, slow burn. The more you fill the basket, the longer the burn.
On my full size UDS with the 18" diameter basket 12" tall, I can get burns of well over 20 hours.
Sterling Skouson