klswift

The easiest way (if you feel a bt math challenged) is to use graph paper. If you have a printer that can enlarge that big, do it and use it as a base. If not, simply start by drawing the center piece from your current block in the desired size on the graph paper and work your way out. Then cut out the graph paper and add seam allowances to the pieces. Don't get hung up on the 12 1/2". You may find out that it is easier to work with 12" or 13" depending on where the pieces land. It is a lot easier to work with pieces that fit on 1/4" measurements than something 3/8" or 7/8". So be a bit flexible with the design. You can always play with borders/sashing to make the exact finished size you want.

Barb in Louisiana

Could you give us a link to the pattern?

maviskw

Lots of people have been giving good answers, but no one has actually answered your question:
"As an example the 6 1/2" block calls for a 1 1/2 x 2 1/2" strip..... does that just become a strip 3x5"?

The answer is "No." They go on to explain why. So 1 1/2 plus 1 1/2 becomes 2 1/2.
And two plus two is not four. It's 3 1/2. And 2 1/2 plus 2 1/2 becomes 4 1/2.
Quilters math? We need to double the base piece without the seam allowance, as others have said.

maviskw

You're on the right track. Two six inch blocks make a 12 inch block. Actually it would be four. Take a 12 inch square and cut it both ways and you have four 6 inch squares. Just add seam allowances as needed.