I tried doing a search here but came up with nothing. But I know there have been photos posted here showing the size differences between O gauge box cars from different makers.
A footnote to consider is that real box cars differed a lot in sizes... just look at photos of real trains. And then there's the aspect of what looks right to YOU. Some folks don't mind running smaller traditional box cars behind a more scale sized loco, but some do.
Then there's also the diameter of track you're using and the over all size of your layout. To me, smaller sized trains look better on a smaller layout, but opinions differ.
The Williams locos you mention are pretty close to scale size, though not as detailed as other high end scale locomotives. And there's certainly no shortage of Santa Fe box cars, since everyone makes them.
The smallest O gauge box cars are the short 027 plug door box cars by Lionel. Close to these are the K-Line 5000 series box cars with opening doors, based upon the original MARX dies.
Next are the traditional 6464 size ones, which is not limited to cars having the 6464 suffix. The now defunct K-Line Classic, and K-Line Train 19, the MTH Rugged Rails box car and the Industrial Rail ones are nearly identical in size.
The Weaver CROWN series of box cars are patterned after smaller prototypes, even though they are scale sized: the woodside reefer, the outside braced box car (SHOWN IN ABOVE PHOTO) and the steel side box cars. Weaver bought the dies from Crown Model Trains (CMP), hence the name. These 3 models are all the same size. More detailed than the traditional box cars (as you can see in the above photo) and a nice size, in between the traditional 6464 and the larger Rail King and Williams box cars.
Next up are the Williams box cars... slightly longer, wider and higher than the 6464 type and certainly closer to scale size, though not detail wise. The basic 40 foot MTH Rail King box car is similar.
If someone doesn't post the photos that were taken and seen here, I'd suggest looking at train videos, taking notice of the rolling stock size. Going to a train store, asking questions and hopefully seeing for yourself (depend what the store stocks) and going to larger train shows that have more dealers and operating layouts.
Once you've been in the hobby for a while, this sort of thing becomes basic knowledge.