CLONE STAMP Tool - I can't change the area of the associated circle of influence

Try resetting the tools and/or preferences.
To restore preferences quickly using a keyboard shortcut:

Press and hold Alt+Control+Shift (Windows) or Option+Command+Shift (Mac OS) as you start Photoshop. You are prompted to delete the current settings. The new preferences files are created the next time you start Photoshop.
Note: Using the keyboard shortcut, preference files for custom shortcuts, workspaces, and color settings will also be reset to default.

from here:

https://helpx.adobe.com/photoshop/using/preferences.html#restore_preferences

Thanks, Bob, for your suggestion. I do appreciate it.

But using the brackets you suggest seem needlessly complicated when I'd prefer to choose a circle of influence (sample size?) of my own choice. [And this I have done umpteen times before.]

My point is:what has happened to the icon that allowed me to choose from an area of any size. Where has the icon disappeared? I have used dozens of times and now it has disappeared. Amazing.

Dera Bob, can you suggest what's happened to the previous choice available? Why has it disappeared?

If you are not seeing the circle change size when using the square bracket keys, then check to see if caps lock is on.

~ Jane

Try using the bracket keys [ and ].