If you selected a Better Bottle, you will have a choice between a 5 and 6 gallon Better Bottle when the project finishes up. You will also be able to choose if you want the Better Bottle with a spigot drilled in the bottom.
If you selected a Big Mouth Bubbler, they all come in 6.5 gallon capacities.
Don't see the answer to your question? Ask the project creator directly.