The large planter should be just right for growth of the Lilly.
You can always choose to go larger on the container size, never go smaller than recommened. The larger the container, the longer you can go before transplanting and the more robust your plants will become. The drawback with going larger is that the container is much heavier as it gets larger. Use the same variety per container. Mixing varities in the same container the more dominant variety will take over.
Small container: approximate size: 8 inches by 5 inches, suited for up to 6 submerged plants or up to 2 bog plants per container
Medium containter: approximate size: 10 inches by 7 inches, suited for one waterlily or up to 12 submerged plants or up to 3 bog plants per container
Large container: approximate size: 16 inches by 7 inches, suited for one waterlily or one lotus or up to 24 submerged plants or up to 5 bog plants per container
*ALL SIZES ARE APPROXIMATE AND MAY VARY