Are there any general guidelines on guessing what the diameter is where a brake would be attached? Lots of 26mm and 25.4mm bars, it seems, are thinner aside from in the middle, is that a pretty consistent diameter for either?

http://www.sheldonbrown.com/gloss_ha-i.html
Sheldon refers to the 'grip area size' but I dont have good intuition when it comes to bicycles, and I dont know what this means exactly....

Since older bars are likely more complicated, I'm inquiring specifically about the latest ones...

Maybe this wont matter really, I just dont know how well a 25.4mm brake would clamp onto 26mm bars.

Thanks!