I will be cleaning and replacing some of the drum parts. So while I'm in there I was hoping someone could enlighten me as to which bearing size this is so I know what axle I have. It is not clear to me even after looking at the following website:
http://www.wildhorses4x4.com/category/bronco_rear_axle
Here are some pics:

I believe it is a 10" drum. It measures 10" from inside edge to inside edge.

Thanks!

You've likely got a small bearing rear end, but it depends on what year your axle is out of. There are some '74 and '75 big bearing rear ends with 10" brakes. Your best bet is to pull the axle and brake backing plate and get a look at the bearing. As long as you've got the drum off, I'd swap out the wheel bearing if you don't know it's history. They're cheap (around $15 each for Timkens), easy to install and aren't terribly safe when they fail. You'd likely hear one, but not always. The PO of my truck installed the seals in mine incorrectly and the bearings were getting very little lubrication. They still rolled fine, but it could have been really bad. The small bearings have a 2.8246" O.D. and the big bearings have a 3.1496" O.D., so they're really easy to tell apart once you get in there. If you've still got the original sticker on the driver's door jamb, the big bearing trucks are marked for 3300lbs and the small bearing ones have either 2780 or 2900lbs (I can't remember). Finally, you can also figure it out by decoding the VIN: http://www.broncohio.com/tech/ebvins.html, assuming the axle under your rig is the stock one.

Axle tube has the big bearing bulge and the 10" brakes make it the...wait for it...medium duty '74-'75 axle.

That would be another big bulge I've got then...

My truck is mostly a '71 so I'm guessing someone before me put in the axle.

I have a 1975 Bronco with that had the "meduim bearing" rear axels with 10" drums.... Its my estimations that these 74-75 Broncos are as good as any later model big bearing axels for larger axels and solid rolled bearings..... Honestly I think meduim is a misconception in my terms.....