All large habitats require 6x6 spaces on your islands.
Using Big Habitats to Level-Up
Habitats can be used to earn XP at a faster and cheaper rate than that of a Farm. If you aren't really into collecting dragons, or just want to level up quick to be able to use items that unlock at higher levels, you can use your disposable income to level up this way. This is best accomplished when you have large amounts of recurring disposable income.
Instructions: Per your level, look for the habitat with the lowest (smallest) number in the "Gold Cost per XP" Column and the highest number in the "XP Per Minute" column. For instance, at level 17 the Big Nature Habitat only costs 1 gold per XP gained, at a rate of 1041.66 XP per minute, over the course of 2 hours. The Big Nature Habitat of course would cost you 125K gold, so if that isn't feasible, look for the next best match. The Big Terra Habitat can be pretty enticing at 5000 XP per minute, but you are paying 10 Gold per XP, and you have to gauge whether you are willing to sit there 30 seconds at a time for long periods of time to make it worthwhile.
Note 1: This process may involve building, then immediately selling/storing the habitats (I'd recommend storing them), as you may run out of space. This process is solely geared toward earning the most XP possible at the lowest cost possible, and is not considering how much each habitat can hold from Dragons.
Note 2: While the Gold Cost Per XP of Big Habitats are less cost effective, the Big Habitats offer lower purchase costs if you don't yet have a large amount of recurring Gold from your inhabited Habitats. Once you have 450K Gold to spend on a frequent basis (and are level 26 or higher), it is recommended you start using the aformentioned process with Light Habitats (NOT Big Light, but the Basic version). See the Basic Habitats tab.