It's gorgeous! Love that pieced border too!
I think I'd put a narrow green border around your small pieced border, then another blue border on top/bottom to make it longer. If you want to make it wider, I'd use the same blue as the last border you used for top and bottom.
Here's a picture to illustrate what I mean about the narrow border on either side of the pieced section. Where the yellow is would be where you put the pieced border.