Description of the need
We've adjusted the size of Backdrop modals a few times, for example #741 and #3297. However I think there are situations where the current modal size still isn't large enough, or dialogs where having the option to go larger could be beneficial in some (but not all) situations.
For example if you're choosing an image from the modal, sometimes you might want to see more items all at once. Or certain Views configuration dialogs have large tables, but that might only be a problem if you have a lot of fields.
Our dialogs are not currently resizeable, because resizing a dialog is highly depend upon the window size (resizing the window resizes the dialog to match). I think it would be helpful to put a "Maximize" icon in the dialogs to allow the dialog to take the full size of the window. I saw recently that AEM (Adobe's CMS) has this feature (third icon from the right in the upper right):
Alternatives that have been considered