Of course. Experimentalists have been shooting electrons and muons and other things inside hadrons since the 1960s (hence the "Decades-Long Quest" in the title of the article). That's how we gained pretty much all of our knowledge of the internal structure of hadrons and how the current theory of QCD was developed.
> What would the predicted diameter be?
A hadron is roughly a femtometer (10^-15 meters) in diameter.
What would the predicted diameter be?