To get a big, separate materials preview window, I used New Window and killed all the subwindows that weren’t the preview.
This seems awkward: my googling indicates there was a preview window in 2009 invoked by ALT+P, but that doesn’t seem to be around any more.
Is there a native way to get a dedicated preview window besides the method I used?
(I want this because the one in the Materials panel gets scrolled off a lot.)
EDIT: I found the rclick “Duplicate Area Into New Window”, which is a bit of overkill (I just want the preview itself), but Good Enough.