You can choose the “Classic or System” themes which have wider scroll bars. Go to "Menu / Settings / Appearance / Themes.
Or if you are using eM Client 8.1/8.2 you can open the “Theme Editor” and customise any theme to have wider scrollbars. “Menu / Tools / Theme Editor”
See @Gary screenshot at end of this thread how to change scrollbar thickness in the Theme Editor.
Note:- Remember If you use the Theme Editor “Save the Theme” first in the Theme editor, by clicking the “Advanced” button at the bottom right. Then “Open and Modify” that theme. The default Theme save directory for Windows 10 is normally eg: - C:\Users\yourname\AppData\Roaming\eM Client