The real determinants for how wide you need to be are: airspeed, bank angle and winds.
The higher the airspeed required in the final turn, the wider you will need to be to make that turn happen, just like in a level turn. A level turn at 300 knots is a heck of a lot wider than a level turn at 70 knots. Tgray would probably know off the top of his head, but the formula escapes me at the moment. I think turn radius is proportional to the square of velocity, though, so a 200 knot turn will be 4 times larger than a 100 knot turn. But I don't have my books with me and I'm pullin' that out of you know where... so take with a huge grain of salt.
The reverse is true of bank angle. The larger the bank, the smaller the turn. Most airplanes I know aim for about a 30 degree bank final turn, but the T-6's and T-37's I used to fly had a 30 degree desired, 45 maximum for the final turn. I think the fighters aim for 45 degrees (but I don't know if they have a max... 60 maybe?). The -135 has kind of an accepted max in the final turn of 30. So that has a big effect.
Lastly, winds. If the winds are blowing towards the runway, you need to be farther away (assuming you are planning for the same bank angles in your turn). If the winds are blowing you farther from the runway, then displace closer.
Now all of this is true if you are planning on making one turn from downwind to final, with virtually no base leg. If you are planning on a significant base leg, then you've got another variable to work with: you can adjust the length of the base leg to make up for any of the other variables.
