Not sure why you are ending up with 6x4??? Cut a 19" piece and fold it in half then you should press a good middle bottom seam. When you make your box corners, the side seam meeting the bottom press mark and sewing scant 3" to each side of the seam should give you a 6 x 6 x 6 with 1/2" at the top (on both sides) for the zipper area. The width of your fabric should be 17" to allow for a 10" width and 3" on each side (making for a 6" depth) with a 1/2 seam. (Whenever I make purses, pouches or similar items, I like to use 1/2" seams because I double stitch for extra reinforcing.) The only thing I can come up with is that you are accounting for the correct size for height and depth, but not for the width. To restate - your piece needs to be 19" by 17". Please let us know what you figured out.