I would use the fabric that is the background of the Death Star. The red binding would then pop. Usually border size are a way of achieving a specific size. Maybe cut 4.5" for a 4" border or wider. Depends on what looks proportional good with the quilt. Really cute quilt.

First off, I love your quilt! Your grandson is a lucky fellow.

I like the look of it with an outside border matching the sky behind the Death Star. I would audition the red binding as well; it's not as loud with the border to separate it from the inner red stripe.

It looks really good without the outer border, as well... if you go that way, I prefer the black binding, or else blue like behind the Death Star.
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Goodness, but you ladies are amazing. Thank you for adding in those borders to the picture. I do like the blue repeated in the border. You have given me more to think about now.