If your turkey is touching the edges of your brining container, that is not a problem. You don't want it jammed in there super tight though. I mix up 2 gallons of brine for turkey. Homemade brine is cheap. I never buy the pre-mixed brines. Some of them are more money than I paid for the turkey! How much brine you need is going to be determined by the size and shape of your vessel, and how much space there is around your meat. If you find you need a little more brine, just mix up a bit extra of just the salt/sugar/water mixture. No need to worry about any additional flavorings you added to your original brine. The salt/sugar/water mixture will dissolve at room temperature with a little mixing, no need to heat.
A 14 pound turkey is going to take a lot more brine than a 14 pound brisket, because turkey has that big open cavity in the middle. So more brine is needed to cover a turkey, than you would need for a solid piece of meat of the same weight.