For longarm quilting, the backing needs to be larger than the quilt top to allow for the loading process, and for room to clamp the sides of the backing (to prevent pleats in the back) and a little extra room if ruler work is being done to allow for the ruler table on the machine.
So, generally my preference is for the backing to be a minimum of 8" longer and 8" wider than the top. (so if your top is 40x50, please send a backing that is at least 48x58) If you are planning to have ruler work done on your quilt, please send a backing that is 10" longer and 10" wider.
My preference is to load the quilt with the seam parallel to the bars, if at all possible, and so that may be a consideration in the construction of your backing. Having the backing seam parallel to the bars can cause less puckering and pleating in the backing, making it easier to manage.
If you are seaming the backing, you may leave the selvedges on the outside edges, but please cut them off of the seam. They won't stretch and flex the way the rest of the fabric will, and leaving them in the middle of the backing can cause issues.
If you are buying backing consider the wide backings available...sometimes it is more economical to buy the wide backing, and there is the added bonus of not having to piece it.
Squareness: it is important that the edges of the backing are square to each other (but it still can be a rectangle). If the backing comes to me unsquare, and I have to take time to square it, there will be an extra charge.
Jenny's Doodling Needle has a great post on Backing Material. and Andi has a great post about why the backing needs to be 8" longer and 8" wider than the quilt.