It's not exactly what you're asking, but one way to increase the size would be to splice the quilt on the diagonals & add fabric that way. By adding 2 high-contrast colors arranged in a triple strip across the diagonal lines of the quilt, it will draw most of the attention to those lines, rather than to the background you say you don't like. There are many different options for that. I played around with some different options in Paint & there were several that worked, depending on how you want the final quilt to look. Here's one example with a navy I pulled from your patchwork & white (I'd go with a snow white so it works with the rest of your piecing):
[ATTACH=CONFIG]558479[/ATTACH]
Or if you're truly set on the name "Plum Ugly" -- you could go with a dark plum instead of the navy.
That would also give you some lovely white space in which to do some really striking quilting & again give a new focus to the viewers' attention. If you do the X, I probably would only add a very, very narrow stop border on the outside edge, just so the beautiful scrappy piecing doesn't get covered up with the binding. I'd probably make it 1/2" wide, so that 1/4" was all that showed after binding. I'd keep it a white stop border & bind in the same color as whatever you use for the X.