Is this the pattern that you are referencing?
http://www.robertkaufman.com/quiltin...moons/#color:1
It shouldn't be too hard to increase the size of this quilt.
However, it depends on how much length (and perhaps width too?) that you are wanting?
I would start by making a line drawing of the quilt, the way it is now.
Then mark the amount of extra length/width that I want to add.
And then ... let the play begin, and try different options, until I get it to what I like visually.