When choosing the width of your wedding & engagement ring, not surprisingly, you need to consider the width of your engagement ring for the following two reasons:
1.   You need to consider the length of your fingers and how much room is available between the base of your finger and your knuckle for comfortably wearing a ring.
2.   In keeping with the strategy of having your two rings look like they were made to be together, you need to consider the proportions of both rings. If the widths of both rings are proportional to one another, the rings be appear to compliment each other better.

Keep in mind, the width of the engagement ring will strongly influence the width of your wedding ring.
 When considering the width of the engagement ring, you should consider the width of the engagement ring on the side or base of the ring, not the top gemstone setting width.

In summary we can as a general guide say, a wedding ring should be either the same width or very slightly wider than the engagement ring band.