Hi, Shani!
The shops at the Resorts sell some larger bottles of water; you'll find full gallons at some of the Resort shops, specifically
BoardWalk Inn and
Animal Kingdom Lodge. Otherwise, the standard size seems to be somewhere around either 20 fluid ounces or 32 fluid ounces.
The
Disney Vacation Club (DVC) markets also sell gallon-sized bottles. The markets are available to DVC members.
One option to consider is to send yourself a box full of water in advance of your trip.
If you send the package far enough in advance of your visit, you can do it at a slower shipping time which will save a bit of money as well.
Simply address the package to yourself at the Resort in question. Include the phrase "Hold for Guest [name] on arrival date [date of arrival]." You could even include your confirmation number on the shipping label for extra security.
When you arrive at your Resort, you can ask at check-in if you have any packages; alternately, you might have a message in your room that a package is waiting for you. Give the front desk a call, and the fantastic Resort Cast Members will have that package to you very quickly.
Have a great trip!
See you later, celebrator,
-Deborah B.