It is common practice to sew two strips together for a sashing or border. Otherwise you would need a length of fabric as long as your longest piece. While having length of fabric is nice and stable for sashing and borders (length of fabric does not stretch the way width of fabric can), many of us do not want to spend the money on the extra yardage.
For those that cut a whole pattern at a time, many start with the largest pieces. Often smaller pieces can be cut from 'left overs'.
If you google 'fabric yardage calculator', you will see that there are several free sites. Then it is a case of entering the data--width of fabric, size of pieces, and number of pieces. This would have to be done for each group of pieces but it would give you an idea of fabric necessary for each component of your pattern.