Can I apply a function to a single site within multi-site?

Hi, not a WPMUDEV issue, so not expecting support, but for the general community I wonder if there is a trick to do this.

What I want to do is apply function to a single site.
I know I can create a child theme applied to just that site and add a functions.php, but that seems overkill, given all I want to do is to alter a specific image size so one liner, just on one site, e.g.
'add_image_size( 'myownsize',111,222);'

Is there any trick shot method easier than setting up a child theme (which isn't hard but adds to maintenance)