I assume you mean finish at 12". The blocks would be 12 1/2" before being sewn together.
The block is made of 4 16 patches. Each of the 16 patches is made up of 2 squares and 2 4 patches.
For a 16" block, the smaller square patches are 2.5" and the larger squares are 4.5", finishing at 2" and 4", respectively.
To end up with a 12" block, the finished size would have to be 12"/16", or 3/4 of the size of the pieces for a 16" block, so the 2" pieces would have to finish at 1.5", so a 2" piece would be used (add 0.5" to the finished size in each dimension). For the parts finishing at 4", they would have to finish at 3", so a 3.5" piece would need to be used.