Screen Sharing opens with random (often small) screen resolution (previously issue #2414437)

Screen sharing from a MacBook Pro Retina to an old Mac Mini. Sometimes the window opens to almost the size of the MacBook screen, but most of the time it opens to a window whose size is about half-height and one-third-width. This window cannot be resized larger than it opens at, and the shared screen can be very small (e.g. almost unreadable). This has been the case since at least OS X 10.6 (Snow Leopard), and is still true in macOS 11.4 (Big Sur).

Here is the workaround: when this happens, in the Shared Screen, open System Preferences -> Displays. For Resolution, choose "Scaled" instead of "Default", then choose an appropriate screen size (it seems to default to 800x600), e.g. 1280x960. The contents of the screen will resize, and you will be able to resize the window. NOTE: Be sure to confirm the resolution change in System Preferences, or it will revert after a few seconds.