Hi Gina. You do need a miniature or a small lily for your barrell. although a medium sized plant will grow in it. The leaves would then hang over the sides and dry out. I have several barrells with lilies of all sizes, and that is what happens when the plants get too big. With one lily you can trim the leaves back, however.
If you put duckweed, azolla, water fern, etc. into your barrell it will cover the surface and may not give the look you want. I have all three, but they cannot be shipped to all states. They can be shipped to TN, however, I live there also.
I will send you some barrel pics on private email for you to see if you want to add marginals (cattails, rush, etc) to your barrell.
Happy ponding!

Please do send me pics.... twnkletoes101@aol.com. Any help would be wonderful. I bought water palm last year, but I can't seem to get it to "come back out". Me and my plants... LOL Thanks Bunches:) Gina