I can't increase (or at least don't know how) the size of thumbnails showing in "File/Image upload" window when uploading content to Flickr, Instagram (via Instagram Studio) and likewise.
I googled it but the answer to "Right click somewhere in the upload window and change the size of...
I started using a Macbook Air 2017 about a couple years ago and have struggled to read and view thumbnails on it from the start. I assumed my eyes were used to the large screen on my Dell laptop and I'd have to adjust. Nope! I have to enlarge the screen to see small print on this laptop still...