If I am measure 17" between my biceps (center to center), about what width should my grip on the bars be (from outside of hand to the outside of other hand)? There must be some rule of thumb for this. Thanks.

For drop bars, yeah (outer ends of the clavicles.) For flat/riser bars, it's pretty much personal preference. You'll see guys on hybrids with sub-600mm bars, and guys on downhill MTBs at +800mm. Whatever feels good.

Bars keep getting wider -- and that's often a good thing. But with gravel bars that flare at the drops, the brake hoods tend to point inward more, which can have a narrowing effect.

So it all depends on what kind of bars and what kind of riding you do -- my shoulder-to shoulder width is about 17.5", and I have a 29" MTB bar, a 42cm road bar, a 700 mm "alt" bar, and a gravel bar that's 42 cm at the hoods but flares out to 50 cm at the ends.

If you're just looking at regular road bars, I'd start with a 42. For anything else, it's probably going to take some trial and error.

What kind of bars?

They are riser bars, but are really almost a straight tube, 31" long, thicker in the middle section where they mount to the stem. I felt like an eagle spreading its wings to fly when I first used them. Yesterday I cut 3" off of each end and I like them much better now. I only ride on paved surfaces. Thanks for your comments.