Thank you for all your replies. I ended up going with a narrow inner border of a goldish-yellow (it looks washed out in the photo, but is a richer colour) and another black border. The next time I'm going to use a bit wider outer sashing and bind it with pieces left over from the batiks. I like the idea of a "rainbow" border.
As to block size, the coloured portion is 6.5x10.5" with two 2.5" wide solids on each side, making a 10.5" block. I like this size as it goes together quickly but looks difficult.
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