They say a picture is worth a thousand words and in this case, I think it's worth more than that!
One of the most common questions is "How much ribbon do I need to make a bow?". It's not an easy question to answer since there are so many variables, but in general, you need about 25 inches of 7/8" ribbon. However, it's really much more involved than that. Below is a graphic showing 12 different bows made from 4 of the most common sizes of ribbon used for Boutique Style hair bows. Each grid square is one inch square.
If you're just starting out, it is easiest to practice with 7/8" wide ribbon. Once you are comfortable with that size you can branch out to smaller widths. If you're stacking bows within the same ribbon width, use the next size right or left to stack with. If you're stacking bows with a different ribbon width, use the bow size directly or above for the best, symmetrical look.