Sraight line measurements (1 dimensional) cannot be figured in square units (2 dimensional). That is area. Do you have a math text? Your calculations for area can be found in there, I guarantee it (as a math teacher
). If you don't have one, check out the local bookstore's Math section. They have quick refresher books on the things you should have picked up in High School...
For the short term, Area of a triangle = 1/2 of any side * the distance measured perpendicularily to the vertex opposite the side a.k.a 1/2 the base times the height.
So, your ship would be 1/2(20)(10)=100 square miles or approx 256,000,000 square meters. It's volume is 100 cubic miles (just multiply by the 1 mile). If you figured 5 city levels in your ship laid out like a city on Earth you would easily have more land area than Los Angeles which currently holds (rather uncomfortably, almost 4 million people.) Rather ,if you assumed 1 human could live and work comfortably in 13000 square feet of space with 10 ft ceilings (a nice mansion for everyone), you could fit over 100 million people on your ship. You might want to scale it down a bit...
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As far as Nuclear detonation goes. (disclaimer: these are quick calculations, there is bound to be at least one mistake)
One megaton = 4.2E15 joules that's 42 followed by 14 zeros.
As a benchmark, consider that the Sun's output (not just what hits the puny Earth, but all of it) is 3.9E26 joules/sec (1 Watt = 1 Joule/sec, think 100 Watt lightbulbs)
Convert solar output to megatons = 9.3E10 (93,000,000,000, I have seen other estimates say 77 billion) megatons each second!!!!!!! One modern nuke is 1 megaton in energy. The tsunami earthquake in the Indian Ocean generated about 475 megatons worth to give you an idea of power. We (earthlings) generate about 60,000 megatons of energy per year (a megaton every 9 minutes (burning dead plants/dinosaurs and playing with controlled (heh) nuclear explosions and the like).
The Sun can make a million years of energy for the earth in one second.
Deneb has a luminosity output somewhere between 60,000 and 250,000 times that of the sun depending on it's
true distance of 1600 - 3200 ly. so multiply 9.3E10 by the low end and you get about 5.4E14 megatons/sec. Heck, wraping your mind around the numbers from the Sun are hard enough. Math and science are just too cool.
Anyway, Selden is right in stating that memorized conversions will be very handy. I suggest you brush up on your basic Algebra and Geometry skills as well, you will find them very valuable as time passes.
