X-pens can make great cages. It should be at least 3ft tall. You might be able to get away with something shorter, but many rabbit will jump out, so a top would be a good idea.
Most x-pens have 8 panels that are each 2ft wide. This means they can be used to make an area at is 2X6 or 4X4 as well as some other shapes that cover a similar area.
One thing I would recommend is getting a step through door. This allows you to open it and go in easily. Most have a door that allows the pet (usually a dog) to come and go easily but you can't easily get into the pen. You can also get ones that don't have a door but the ends aren't attached, which these you can use swing open the end and use that as a door. This can also be done with other pens, but you may end up having to take it apart a bit.
Don't buy new unless you have to. You can find some cheaper ones for sale if you look and are patient. Some can be as cheap as $20 compared to the $60-100 you could pay retail.
If you are going to use it as a cage, then it is a good idea to have a floor of some sort. This would protect your floor from pee and chewing as well as being easy to clean.
