My personal preference is 5" per side to allow room for:
1. Attaching to leaders and clamping sides
2. to allow room for an out of square quilt top
3. To have room on the side to test tension
4. If using my ruler base I need the extra room on the sides so my ruler base doesn't interfere with my side clamp. In cases where I have received too narrow a backing I have to remove the clamp while quilting close to the edges.
That said I have had plenty of backs come through that give me MUCH less. I think the least I had was 2" per side. On that one I was forced to sew a strip of my own fabric to the back so I had room top to bottom for my leaders as 2" was just not enough and the sides allowed me darn little wiggle room. I did it but I was not a happy camper.
Contact your LA quilter and ask what her preferences are. In most cases it is fine to simply sew some strips of muslin or any scrap fabric you have to get up to the needed size for attaching to leaders and side clamps. Just make sure you tell your quilter that you did that only to get the backing up to size and you don't want any of it incorporated into the finished quilt. I think all but the very greenest of LAQ can accommodate that.