Could you provide us a screenshot of the recording to know your situation clearly? For security purpose, please remember to cover some private messages.
Which type of screen sharing do you use? Desktop, Window, Whiteboard or PowerPoint?
After the recording is finished, we cannot modify the screen size to the original size. It recommends you use desktop sharing because only desktop sharing could show full screen of user.
As a supplement, there is a red box around the sharing screen. The red box is the border of recording. You could determine if this is the suitable size by the red box.
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