If you lay out a sheet on your bed and pin it up to the size you think you want your quilt, you can figure out where you want the edges to fall, and then measure. on a 4-poster you might want it wider than long!
A couple tricks I have used are to add an internal border strip at the top and bottom, interior to the outer border,- either plain for pieced in a similar way to look nice with the rest of the quilt, and to vary the sashing widths and outer border widths to make up the difference. On one medallion I made the center part oblong and then just played along with it so it came out oblong in the end.
I don't mind square quilts anyway - less likely I have to worry about which end it up!
Don't be so hard on yourself!