The current pattern is 8"X10"X4", you said you wanted it to be about 15". I'm assuming (maybe incorrectly) that you want the original 10" dimension to be enlarged to 15". Do you want all the dimensions enlarged proportionally? So the new bag would be 12X15X6 or do you just want one dimension enlarged??? makes a big difference in how you would enlarge it. Also if you just enlarge the pattern with a copier, you need to remove the seam allowance first enlarge it then add the seam allowance back on.