Originally Posted by GingerK

Well, lay out a couple of your blocks. Now imagine sewing sashing to the left and right side of the first block, then to just the right side of the next block. These two blocks can now be put together and the sashing will be between them. Once a whole row is done this way, you can measure and cut a long piece of sashing for the tops and bottoms of all the blocks in the row at once.

As for the length, the sashing to the left and right of the blocks would be the same as the length of the block.

For the sashing above and below the ROW of blocks, add up the FINISHED width of your blocks plus the FINISHED width of the sashing between them plus 1/2 inch.