On the site WeTransfer (cloud-based file transfer service), on my Mac, Safari and Firefox render the interaction box size differently (smaller) than Chrome and other browsers. And I think smaller than intended.

     228×334 px  vs.  280×410 px

As a consequence, the line “xx minutes remaining” while uploading is covered and not visible, so I am missing out on useful information.
I’ve checked my zoom settings, but those seem default, and zooming doesn’t alter which text is visible anyway. I’ve checked (tried without) plugins and extensions, but that didn't change the box size. As far as I can decipher the CSS, the box dimensions seem to be defined in em units, thus relative to the type size. I don’t think Safari has a user accessible default font size anymore (only an optional minimum size)? One thing I can’t easily check, is a different computer without any of my personal settings. I can’t be sure if it isn’t a local setting that is causing this, hence my question here to see if some of you have a few minutes to spare to check what sizes of boxes you get in Safari? I.e. is it just me?

It's not clear to me if you've looked at Safari > Prefs > Advanced > Accessibility where you can change Safari's font size.

I don't think that Safari Prefs let you choose the default font size, only the minimum. You can, however, change the default font size via the Safari tab of TinkerTool (free).

I have. Boosting not the default size, but the minimum size all the way to ‘18’ would make a difference on that site. But that is tangential and not practical.
My point is different though: If it is just my setup, then I should fix something. If it happens for many others as well, then I’m going to ask them to fix it.
So what size box did you see?

Thanks! That was the clue I needed. I had changed the default myself (probably with TinkerTool many years ago when the prefs setting was removed), and forgotten about it. Undoing that setting does indeed bring all browsers to same size renders of that site.
Much appreciated.