You want to leave enough room on the sides to make it easy to clean and not make it look crowded. I wouldn't go over 42" width if you center it on the wall. Centered looks nice if the sides of the vanity are worth looking at. For example there are many furniture look vanities on the market today that only look good centered on a wall so you can appreciate them from both sides. Without pictures or a floor plan I can't recommend centered or up against the wall.