If you must have a border for size, consider a nice warm brown, like milk chocolate?

If the size is ok, I would simply bind it. I would probably use a darker color, maybe a dark brown or green.

I, too, am a "border person". I think a small medium to dark border would frame the quilt and not make it too much bigger.

I might put something fairly thin in a dark blue or black, just as a frame. Or, maybe a black thin border and then a rainbow on black. But I think it's fine as it is. Very nice.