Arrowsare "Supposed" to be whatever you need them to be based upon your choice in rest combined with how that length and your desired tip weight will affect the arrow spine of the shafts you want to shoot. The length and the weight of your components as they relate to your bow are more important from a spine perspective than they are from an actual "right or wrong" length.
Even your choice in fletching and nock components have an effect on how long a desired shaft should be for your set up because they affect arrow spine from the opposite end.

More weight up front "softens" arrow spine, weight in the rear "stiffens" arrow spine.

How complicated ya wanna make this? LOL

1. Decide what length you want based upon your actual bow and rest set-up (actually measure)
2. Decide what weight head you want to use, and style fletching
3. Decide what you want your finished arrow weight to be based upon how fast you want your bow to shoot
4. Choose the appropriate shaft that will be spined correctly for your bow and give you the finished arrow weight you want at that length and with those components