I have a UFO Bargello and the advice I rec'd from the designer to make a wall hanging size into a lap quilt size was to make my strip sets a little wider than the pattern calls for which will add length. To add width, I think I would suggest making to top the size of your pattern ... then adding strips on each side adjusting your strips to follow the pattern. What I'm trying to say (not very well) is ... on the left side of quilt top to add a row start with the color of your second block from the 1st row. Do the same on the right side with whatever the second clolor is on the last row. Do this until you get your desired width. Does that make any sense?
I guess too it depends on which Bargello pattern you are using really.