The colors of your fabrics are really pretty.
I like your idea of making/separating some of the blocks.
If you want to "box" just one square with one fabric like you have pictured, and have it end up the size of the four other blocks sewn together, you will need to cut them these sizes:
3.5 x 6.5 and 3.5 x 12.5 inches - if you only want to use one color.
When you sew the four 6.5 inch units together, The block should measure approximately 12.5 x 12.5
You could make this top as big as you want, and then you could use the "extra" units to make a lap quilt or a "topper" or for pillow shams.