As mentioned above, start by measuring the space you have to work with, even if you have to move some furniture around to make it work. That will tell you the size you can look for.
Christmas trees are typically labeled by height. That's why it pays to measure the space, both in diameter and height. You can find trees anywhere from around 2 feet to 9 feet or more. If you plan on a more understated style or you have less space to work with, choose a tabletop model that's around 2 feet. 9-foot or more trees are good for larger living rooms with vaulted or otherwise elevated ceilings. The most common height you'll tend to see is a 7.5-foot tree.
It's also important to consider the diameter of the tree. This will most directly be influenced by the height as well, but even trees of the same height can have different diameters depending on how textured and bushy the tree is. Look at the specifications of the tree, which will often give you a measurement like the circumference. Some may even tell you outright that the tree has a certain depth measurement in the space, like 5.5 feet.