whoa. you plan on doing your mobo wires too? good luck...
1/4" for the 4-wire ones, even the 6 wire if you have it.
1/2" to 3/4" for the 20-24 wire atx connector
1/8" for 2 wired ones.
here's my recommendation for cutting the flexo pet sleeves: cut it with SHARP scissors, if you dont have sharp ones, buy new ones. then insert something like a screw driver bit or something wide and then use a soldering iron to seal the individual V shaped points. if you really KNOW how to use a hot knife, and have one do so. using an open flame or other methods can cause all the V shaped points to be sealed together. this causes problems since you cant open it back up. i'll see if i can do some pix of this method.
use heatshrink wrap to seal the ends of the sleeving. the higher the shrink ratio the better.
www.svc.com has good prices on the sleeving for the lengths you get (ie, to match their prices per foot, you may have to get 100' elsewhere) and 3:1 ratio heatshink tubing. 2:1 is what you would find elsewhere. 3:1 means a heatshrink tubing piece with 3/8" diameter will shrink to 1/8" after heat is applied. apply the heat with a blow dryer (heat required, so if your BD has a cool mode, dont use that) and careful not to blow on the flexo pet sleeves themselves, they will warp.
www.svc.com sells in 5' lengths, so get 10' of the 1/4 and 1/8 just in case.
DO NOT PUT OPEN FLAME TO ANYTHING. the wires, heatshrink tubing and the flexo pet MAY NOT be flame retardent. just put the stuff near it, not in the flame.
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http://www.mdayx.com/articles/sleeving/sleeving.htm
there are some pix on the bottom to show you now to cut them.