What is the best size aquarium? The largest one you can afford of course!
A larger aquarium allows you greater flexibility in the number and type of fish and invertebrates you can house. More importantly, a larger aquarium is more resistant to quick changes in water chemistry as well as changes in temperature. These are not minor benefits when you consider a sudden chemical imbalance can potentially injure or kill the inhabitants of the tank.
Some fish species require a significant amount of room to swim. Fish such as tangs and large angels are adorable when they are 1" juveniles, however grow to be 12"-24" adults in a short period of time. If you intend to keep these sorts of fish, it is important to choose an appropriately sized aquarium based on the fish's ADULT size.
While a larger tank has overwhelming benefits over a smaller one, cost is a major consideration. Complete reef aquariums cost an average of $40 per gallon. Some of the more esoteric systems can cost over $100 per gallon. As you can imagine, a reef aquarium can get unaffordable quickly, so it is important to select a size that realistically fits into your budget. Whether you are looking to purchase a small tank or a large one, there are certain factors to consider regardless of the aquarium's size.
First, how wide is the tank front to back compared to the height? Ideally, the tank should be wider than it is tall. A wide tank maximizes the available surface area for gas exchange as well as providing greater flexibility for aquascapes. The standard sizes available today seldom fit into this category since most are taller than they are wide. For those looking for a smaller aquarium, take a look at a 40-gallon breeder (36"l x 18"w x 18"h). For those with a medium sized setup in mind, a 120-gallon (48"l x 24"w x 24"h) is available. The most popular large size tank is the 180-gallon (72"l x 24"w x 24"h). Custom sized tanks are available through various sources in both glass and acrylic. If you are not satisfied with the dimensions of the standard size tanks, there is always the option of a custom manufacturer.
A very important element to tank selection is maximizing your equipment. Lighting and filtration can be quite expensive. In some elaborate setups, the cost of equipment is far more than the cost of the tank itself. Provided there is enough space in the room, you will want to get the largest tank your equipment can support. For example, if you are going with a 48" lighting setup, you could install the lights above a 55-gallon aquarium, or a 120-gallon aquarium. They are both 48" long. With the 120, however, you are getting more than twice the volume of water and much more desirable dimensions for both aquascaping and gas exchange.