I've not tried to see if 15x15 works but I think you end up with dark corners with that.
Well, you end up with a lot of unwanted stone and a lot of mining work if you do it like that. My technique is:
digs out 1x9 squares from the central torch in all 4 directions
Place a torch at each endpoint
continue from each torch in all 4 directions
If you find ore, dig that out. if not, leave the stone where it is (if there is an ore vein, you will find it with this technique, no need to mine all that stone in between).
Of course, this needs more torches. But torches are basically free.