I haven't done a keyhole quilt but been considering it so did a bit of quick number crunching. I worked out that if you did your blocks to finish 6x12" you can get 40 blocks, ie the number of strips, depending on how you rearrange the colours to get the centre contrast. With that you can do a layout of 8 across by 5 down to get a top that finishes 48"x60". Add a border, say 6" and you get a 60x72 quilt.