Please keep in mind that there were absolutely NO engineering/design "best practices" used in the development of this project; a lot of it was arbitrary, and I made it up as I went along...
Once I decided on a build volume (in this case, X 12", Y 18", Z 18"), I printed the gantry bits and test fitted them with an oversize length of Makerslide. This process gave me a rough idea of the PVC lengths required. There is a lot of wiggle room allowed, and I wasn't too fussy about the actual final build volume.
A rough measurement of my printer uses PVC of the following lengths: X = 23 1/4", Y = 27", Z = 32 1/2" (corner pieces), Z = 24 1/4" (internal lengths for the Z axis Makerslide).
For best results, source all of the PVC from the same manufacturer or you'll go crazy compensating for slight diameter differences. Print one or two test pieces, and adjust the print size for a nice fit...you don't want it too snug.