Fuel Storage is both the name of research and ship design items divided into various sizes.
Fuel Storage - Tiny : 5,000 litre storage adding 5 tons to a ship. Fuel Storage - Small : 10,000 litre storage adding 10 tons to a ship. Fuel Storage : 50,000 litre storage adding 55 tons to a ship. Fuel Storage - Large : 250,000 litre storage adding 170 tons to a ship. Fuel Storage - Very Large : 1,000,000 litre storage adding 1065 tons to a ship. Fuel Storage - Ultra Large : 5,000,000 litre storage adds 5185 tons to a ship.
The smaller tanks will generally be added automatically when you choose your deployment time. The longer the deployment time the larger the fuel storage will need to be. Larger storage is generally used for fuel tankers or sorium harvesters.