From a piece 6" x 10.5" you could cut 2 charm squares (5") or 2 strips 2.5" from which you could cut 8 - 2.5" squares or leave them as strips for jelly roll type projects. You could instead cut 3 - 2" strips which could be cut into 2" squares or left as strips. There are many combinations of triangles and other shapes that could be cut from that size piece.
In a colorwash project variety of fabric is more important than repeating the same fabric over and over. I do repeat some fabrics 2 or 3 times in a piece the size I built yesterday (256 squares) but I probably have 230 different fabrics in it.
I work with 2.5" squares and sew them together in rows like I showed in this post. A lot of people would rather use the 2" gridded interfacing and use 2" squares to build a colorwash. Here is a post where I show how I store my squares to make it easy to see all of the fabrics at once.
This link from my Label list (right side bar) will take you to all of the posts that refer to colorwash.
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