Aside what's comforable to grab with one hand to keep you from becoming the honoree at a Memorial Servies...

I don't know if there's a "hard and fast rule".

I believe that wood hand (safety) rail's cross section is a function of a number of factors. First, you have the issue of being able to span between the supports with something that will not deflect when you stand on it (or break under shock loads). The width of the "rail" section affects the ability to stabilize the rail in normal mounting situation.

That's why I think that most common situation is about an 1" wide by a little more than 1" vertical (before rounding) sections.

Upon reflection, I think that going much bigger is going to make grabbing with gloves less secure.

The OSHA or ADA handrail really is intended principally as a support -- not something that is a grab rail.

If you're a gymnast that swinging around a bar, you have a function of something that you might have to "find" at the end of flip, or whatever -- you wan't something that takes shock loads with a larger span without too much spring, etc.

You have loved ones with smaller hands who might want to hand on for the lives, too. ;^))). You have pipes available -- see which diameter you'd like to hang onto for your life. I like the bigger looks too, especially on a large boat; but as a grab rail (not a balustrade) I think you get a different answer.