I asked this elsewhere in the forums and haven't gotten a response, but in one of the WWDC word clouds I saw the label "Extra-small widget option in visionOS", but looking at the WidgetFamily it doesn't look like there is any new size that has been opened up for 27.
Is this a new size to be supported or something else entirely?
The new sizes on visionOS will be .accessoryCircular and .accessoryRectangular. https://developer.apple.com/documentation/widgetkit/widgetfamily.
They're currently unavailable in the visionOS SDK, but will become available in a future seed. If your app is an iOS compatible app (built with the iOS SDK but running on the Vision Pro), those sizes should already be available in visionOS 27.