bearisgray

If you want to make life easier for yourself, get the wider backing.

I prefer to use the French fold type of binding. I also cut my bindings on the bias around 3 inches wide and they finish at approximately 1/2 inch wide. There are lots of videos available for it. I do a bit of a practice on a mini quilt sandwich to see if I like the effect and to get the settings right for this project. Also to practice on mitering the corners.

Search for: French fold binding for quilts

How much extra you will need for the backing depends on how it will be quilted. If I'm doing it on my machine, I can get by with about 1.5 inches around the edges.

Another thing you could do for the backing is to insert a contrasting strip for more width. I would suggest putting the strip towards one side so it looks intentionally off-centered, because it is difficult to line up a center strip so it is exactly centered on the top.

I trim my quilt sandwich before putting the binding on, and then I stay stitch the three layers together (about 1/8 inch from the edge) so I only need to worry about "one" layer when adding the binding.

Some people trim their quilts after they put the binding on.

I pin basted my quilts ad then did stitch in the ditch and straight line quilting with my Pfaff 1471 (1980s machine).

If you are having this sent out to be quilted by someone else, check with that person for his/her requirements for backing.
As far as I know, longarmers want the batting, top, and backing separate, because they load the pieces separately??