The larger the better. These are not suitable for Niles. Even 55 gallon aquariums will only work for a short period of time. Eventually you will need to buy or build a custom enclosure for your Nile. For those of you who can, dedicating a full room in the house for a Nile is a good idea.
Niles are aquatic in nature.
Their tails are evidence of this, and have evolved to assist Niles in swimming. The large keel serves as an excellent swimming tool. Many Nile owners provide large water containers for their monitors. Pharaoh and Delta have a 90 gallon horse trough in their room for swimming. Niles will soak and swim in the water container, and may also use it to hide when threatened.