What size tank do you want?
The biggest one possible, right? Well it's not that simple... Water weighs 8 pounds per gallon, or one kilogram per liter. A 100 gallon tank will weigh about 1000 to 1200 pounds once sand, rock, sump, stand, and its contents are taken into account. Floor strength needs to be taken into account, as well as location because once it is filled, it will be difficult if not impossible to move.
For the purposes of this document, a large tank is considered to be any tank that is 150 gallons or greater. At 150 gallons, it can be assured that entire setup should weigh over one thousand pounds (450 kg).
As tanks increase in size, certain factors that were negligible for small tanks begin to appear. One of the most noteable is humidity. Large tanks evaporate a considerable amount of water. This water may find its way into your walls or ceiling which will eventually cause structural damage.
It can be said with great certainty that as tank volume increases, the price will increase exponentially.
Which tank shape and dimensions should I pick?
Reef tanks come in all shapes and sizes, but the most common are rectangular. Hexagonal, cylindrical, and even spherical tanks can be found, but I suggest leaving those novelty items for the terrariums and goldfish tanks. Oddly shapped tanks are difficult to clean in a marine environment.
In the U.S., tanks are often sold in terms of gallons, shape, and whether they are "tall" or not. For example, a 120 gallon rectangular tank may have the dimensions of 48" x 24" x 24" (length, height, depth); however, a 120T (tall) gallon rectangular tank would have the dimensions of 60" x 24" x 18". These have the same volume but drastically different dimensions.
If you are planning a coral reef tank, the most important dimension is the least obvious: depth. A deeper tank yields more aquascaping possiblities. Most experienced reefers if asked what they could change would say that they wanted a deeper tank. At a minimum, twenty four inches should be reserved for depth on reef tanks larger than 75 gallons.
In case you were wondering, the least important dimension is height. In fact, the taller the tank, the more expensive the materials are to build it. Tall tanks require thicker tank material in order to prevent bowing and rupture. Tall tanks also require more powerful lighting in order to get the best penetration to the bottom.