Bicycle Mechanics - What size road bike levers to get?

So I have drop handlebars that are thickest at the center where the stem would be and gradually a lot thinner and flat at the top and then a little thicker and round at the drop. Since the thickness is not uniform, what size levers am I suppose to get? I want to get brakes for the top of the handlebar, not the drop.

The handlebar clamp area is either 26.0 or 31.8 mm, then it goes to 23.8 mm where the brakes attach. Since nothing attaches at the tops, there's no standard dimensions if they flatten.

In-line/interrupter/cross levers are designed for one of the above dimensions. Without seeing your bars, my guess is that you'll need to install the levers near the stem and it's probably 31.8 mm. Measure it to be sure.

+1, Road bars are 15/16th, where the brake levers go, on the downward bend, center as you note, is bigger..

if you wish to get interruptor second levers installed, inline, they will tend to fit at the larger center diameter..

if your road bars have an unconventional flattened shape on the 'tops', then its quite possible they aren't compatible with CX interrupter levers at all. hopefully the larger diametr round part thats in the stem clamp extends far enough to either side for said CX levers to fit.

if your road bars have an unconventional flattened shape on the 'tops', then its quite possible they aren't compatible with CX interrupter levers at all. hopefully the larger diametr round part thats in the stem clamp extends far enough to either side for said CX levers to fit.
Several models of interrupter levers are sized to mount at the center bulge diameter of either 26.0 mm or 31.8 mm and immediately adjacent to the stem. A few have 23.8 mm clamps (the diameter where the brake levers or brifters mount) and those won't fit bars with flattened "wing" tops.