I love the Dresden Plate Block. What you can do is alternate a solid square between the Dresden Plate Blocks if you want to make it a king size. When I did one, I did Stitch in the Ditch on the plate and a flower design in the middle circle.
When I came up with an odd # block (you said 21), I cut the extra Dresden Plate into 4's and used them as corner stones in the border with the end of the blade pointing toward the quilt. It turned out well. Good Luck.