Heres how you adjust the Scroll bars for Windows 11
Press the Windows and R keys to launch Run.
Type-in regedit.
Navigate to HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics.
On the left hand pane, double click on ScrollHeight and ScrollWidth change the string value to edit the height and width of the scroll bar. (Default value is -255)
Keep in mind that the formula is -15*desired height/width in pixels. (e.g. -15*20 = -300)
After setting the desired height/width, close the registry editor and sign out and sign in again from your computer to apply the changes.
Result
Default value is set to -255 and I set to -400 and it widens after sign out and sign in
(Theres also a Color folder that has Color for Scrollbar in RGB Color)