In the rail fence pattern, you can make your block about any size you would like it to be, BUT it is always a square. So, it would be say, 9 12/ X 9 1/2, not the measurements you gave which make a rectangle not a square. If all the blocks are that size then 5 acrossed would measure 45 1/2 inches the same for 5 blocks down. Then add the 2 inch border is would be 47 1/2 X 47 1/2.
If you are going to make the blocks 12 1/2 X 12 1/2 then 5 down and across would make the rows 60 1/2 X 60 1/2 add the 2 inch border and you get 62 1/2 X 62 12.
Does that make sense to you?
Of course if you want the quilt to be a rectangle instead of a square you need more rows going in one direction, say, 5 blocks across and 7 blocks down. Then you would get ( with a 12 1/2 inch block ) 60 1/2 X 84 1/2 and then add the border.
Don't know if this helps, but I tried.