Thats not quite convincing, I dont think you can make that generalization based on 4 or 5 limited cases.
Oh, there are plenty of other cases, believe me. Take a look around your Mac. The point was not to show a comprehensive list, it was to show they do exist (some evidence is better than no evidence at all). Think back to when I listed Windows users who liked Safari, the point was to show that this type of user exists.
My job is not to convince you, only you can convince yourself. I'd suggest anyone reading to have a look round for resizable dialogs the future. You may find them if more places than you realised, but never noticed before — they can be really useful.
Or you can clarify what exactly do you mean by "dialogs", since its quite a confusing term.
Ask Erwin for their definition as well, as they originally talked about dialogs not being resizable — Erwin?
I'd take it to mean any UI Element which is not part of the main application or document (if it is a document based application) window (whether modal, non modal, or a sheet) which either requires (sometimes optional) action from the user or allows for user input.
I agree it is quite hard to define, but it is easier to work with examples. So I'd say the browser window in Safari is a window, the application preferences are in a window, but the open and save sheets are both dialogs and some of the options you access from the preferences (e.g. the local database manager) are dialogs.
I'd say sheets are a way of displaying a dialog which is relevant to a particular window, so any sheet is a dialog. It is modal is so much as you cannot access the window of the sheet, but it is non modal is the sense that if you have other Windows open at the same time you can access those — best of both worlds.
Of course not all sheets are resizable, many don't need to be (do you want to save this document…) but the ones which would benefit from being resizable generally are.
See
this video for examples (It uses your favourite web browser — picked it out specially

).