I believe Connecting Threads always mentions how much fabric is required for the backing on their kits. This makes it easy to order a coordinating fabric in the right amount for the quilt kit.
Alternatively, you can figure out from the finished dimensions of the quilt how much fabric will be required for the backing.
Many quilters wait until a top is finished before going shopping for a backing fabric. The backing fabric does not need to be part of the top. However, for convenience, you may want to order coordinating backing fabric at the same time as the kit. If you cannot easily find the information online, you may want to email or call the company to ask what coordinating fabric is available for the backing and how much you will need.