If you're designing your own, I did it without a design wall. i drew the basic shape on a sheet of graph paper, and decided how wide the strips should be - I used 1", 2" and 3". i took another sheet of graph paper and colored stripes on it, similar to the colors of the fabrics I wanted. Then i cut the colored strips perpendicular to the changes in color and taped the colored strips to the shape I wanted on the paper. To make it easier to piece accurately, I then labeled the columns of strips with left and right for the direction I wanted to iron, so that the columns of strips would lock together when pieced.
I haven't done a bargello with a mirror image effect, but that would be designed in halfs or quarters. Neither have I done one where both dimensions change width.