Dimensions- The surface area of your aquarium is important because it will contribute to proper oygenation and helps you determine how and were to display the tank for the best view. Height needs to be considered for placement as well, but is also important because many marine and fresh water environments can benefit from having taller aquariums and different "levels" for different species of aquatic life.
Weight- A general weight estimate is about 10 pounds per gallon for a complete aquarium set-up. Ensure that the stand you will be using can accomodate the total expected weight. Most household furniture is not recommended to hold an aquarium.
Sizing- Most people are limited on aquarium size by where they intend to the place their new set-up. In general, it is best to select the largest size you can for your space. More space will allow for a more stable aquatic environment and enables you to choose from a larger variety of fish and plant species.
Aquarium Type- There are two materials used in aquarium construction, acrylic and glass. Glass is preferable as it will better resist discoloration and scratches.