Sherry,
I always design my own quilts, rarely ever buy a pattern, just graph them out on 4-squares-to-the-inch graph paper when I see something that gives me an idea.
I also make the blocks before I ever cut sashings, or borders, or setting triangles, and then put those blocks up on my design wall. When my quilt has reached the size it wants to be, it tells me by letting me have no more fabric, or ideas, or time, or even interest! And then I border it, finish and bind it. (Or maybe not...sometimes even though your quilt isn't finished, you are finished with your quilt!)
I guess what I'm saying is, quilts are not often EXACTLY the size we thought they'd be. And it usually wouldn't matter if we'd let it be that way. Have a general idea of what you want to make and then enjoy the creative process! Can't wait to see a pic.
Jan in VA