First, are you really sure you want a Nile? I appologize if you are already aware of the facts and want one anyway, but Niles get very large and very powerful. They are fast, strong predators equipped with a wide variety of weapons. Of all the monitors, they are also the most paranoid. They almost never come to trust humans, remaining terrified of us all their lives and more than willing to defend themselves from our percieved agression by biting, clawing, tail whipping, and deficating all over us. Even if you do not want to handle your pet, their sheer size makes them a nuisance to care for. As adults, they require huge, sturdily built enclosures (they can rip through most materials in time). They are semi-aquatic, so a very large tub of water is a must. They tend to deficate in their water, with poo that quickly clogs any filters. Changing this much water on a daily basis is a serious chore. If it ever gets out, it can cause an amazing amount of mess and property damage in a short time, and will likely wedge itself in the most innaccessible place you'd never have thought it could possibly get into. Getting it out requires both strength and heavy armor. I speak from experience on these things. Niles are impressive, beautifully marked and hardy, but are a real pain in the butt and will result in a fair amount of lost blood (not to mention sweat and tears).
That said, a small (5 foot) individual could probably get away with an enclosure as small as 10 feet long by 5 feet wide, with 2 feet of dirt to burrow in and an additional 2 feet of clearance above that. Larger individuals (up to 8 feet) really need a whole room to themselves.
Good luck, whatever you decide.
Luke