mjsylvstr, it is beautiful! thanks for sharing the picture with us!

Sounds like a 4 patch pandemonium.

Stack n Whack has always seemed to have been misnamed to me - because one lines up (stacks) the pieces of ONE fabric carefully and cuts carefully - attempting to have each piece the same in a given pile.

Seems like similar techniques/patterns have a lot of different names.

I have seen a video of it. I think either on the Quilt show or an old Simply Quilts. I believe what you are talking about is you take a stack of squares, the size doesn't matter, as long as they are all the same size.
You then slice the stack of squares into pieces. You can make straight cuts, but it works best if you make angular cuts. If you are cutting a wonky 9 patch, you would need nine squares. If you cut a wonky 4 patch you need 4 squares.
You then sew together the pieces you cut - one from each color square to make a block. You then have squares that are identical in shapes, but each one is different colors.
Make sense?

If I want to make 12 1/2 ", how big should I cut my prewashed squares?

That is what I did, but there is a special method for moving the cut pieces. It isn't just move one from the top to the bottom. With every cut you move a different number of pieces to the bottom so you get a true mixture of fabrics in each block. I did the magic tile block with the "grouting" between pieces. I didn't make all the cuts at once. After the first cut and shuffle, the pieces are re-assembled with the 1 1/8 strips between and then the next cut is made and shuffled. It's the fastest quilt I can make.

Do you mean that you added the sashing to the cut piece before re-assembling the pile and cutting again? So that the finished item would have the stained glass window effect (as MelodyW, Thequilter48 on page 4 of this blog has done?) (Sorry to be a bit dense!).

There is a tutorial on UTube showing how to make Crazy shortcut quilts. That's what you are describing. The lady is Marguerita McManus and she and her daughter wrote a book with the instructions on how to make these quilts. They are quilt as you go and are very easy. Beautiful!