Are you talking about enlarging a 6 inch finished size to a 12 inch finished size?
If so, this is what I would do with the 1-1/2 by 2-1/2 inch piece"
As Kassaundra already mentioned -
In the 6 inch block, the "finished/showing" size of the piece will be 1 x 2 (cut piece minus seam allowances)
In the 12 inch block, the "finished/showing" size of the piece will be 2 x 4 (doubled in both directions)
Then the seam allowances are added back on, so the cut size of the piece will be 2-1/2 x 4-1/2 inches.