Configure thumbnail size in grid layout:
Click the library options ellipsis.
Select Edit Thumbnail Settings.
Click the Grid Layout tab.
Select an option from the Thumbnail Size dropdown.
Click Apply.
Configure thumbnail size in list layout
Available for image and video libraries only.
Click the library options ellipsis.
Select Edit Thumbnail Settings.
Click the List Layout tab.
Select an option from the Thumbnail Size dropdown.
Click Apply.