You will need to install a user script manager extension to install this script.
Late february 2020, google images stopped displaying image size over thumbnails.
With this fix (if the extension managing your styles can handle it) you can choose if the size should always be displayed, or only on mouse-over; and if it is displayed at the top or bottom, on the left or the right, of the thumbnails.
Version 0.4.1 adds new options to style the size caption.
Version 0.5 adds an option to align on the right.
A simplified version of this stylesheet is available on userstyles.org
For more userscripts, styles, extensions, tips, ..., visit my programming page.