The first basic rule: your short end of the frame will cap your longer ends.
What do you mean?
If you are building a frame that is 4 feet by 8 feet(4x8), your 4 foot boards will cap your 8 foot boards, or in this case, 7'9".
The reason the board is 7'9" and not 8'0" is to compensate for the 1 1/2" from each 4' board on each end. In combination, they add up to 3" and therefore subtract that 3" from the 8'0" boards. Therefore: 7'9".
You want to make a perfect 4x8 frame.
However, it is not always the case to have your short ends cap the longer boards. In some cases, it is wiser to have your longer boards cap the shorter ones. When your longer side is expected to hold weight, the longer boards should cap the shorter boards.
The reason for this: You want the weight to push against the boards and not burden or stress the screws that are connecting your boards.
Good luck with your projects!