I see some rifles in .338 winchester magnum are provided 24" barrels and others are provided with 26" barrels. Is it important to select the 26" barrel? Is there a place to look up data on the difference this makes or some commonly used rule of thumb?

The 338 win mag and similar magnum rifles are typically chambered in a 24" tube, the 26" tube however will allow you to achieve some more velocity with slower burning powders. For most applications i would say that the 24" would be just fine.

I have a 26" tube on mine.They say 50-75 fps per inch.

That is cheap and easy performance to pick up, IMHO. I like the 26's.
[If I need a "brush gun" I'd go for a Marlin Guide Gun in 45-70]

EKM

I would go with the 26" barrel I like to suck all the preformance out of a firearm safely I can. Good luck.