I think off white/cream would be fine.
As far as the size goes: If you take off some of the existing quilt and add it to the sides then add contrast borders it could turn out quite nice.
Something like this where the black is the original quilt with some length moved to the side and the red area is the 'new' border area.
[ATTACH=CONFIG]394660[/ATTACH]
After rearranging like the above, you can add more borders to get it to a size you'd like.