I'm not entirely sure how to describe my situation, but I noticed that for a while now, whenever I search things up by some tags. The images displayed are usually displayed in a 5x5 fashion, 5 pictures across, and 5 down, but I recently seen it turn into a 6x6 which annoys me. 

I'm trying to take a screenshot of full pages and the 6x6 makes the pictures seem too small, so I don't know what to do. Is there a way to revert this? What is this called? 

Zoom in? if you are on PC your zoom level changes the grid.

There is no fixed pattern for how the thumbnails are arranged. The thumbnails are a fixed size, and will fill as much horizontal space in your browser's viewport as they can. There are no site settings that will affect this. It's possible that there are settings in your browser or on your device that affect the scaling, but you'll need to contact their support for help with that.

By the way, this isn't really an answer to your question, but there are always 42 thumbnails per page. 42 is not a square, so the number of thumbnails horizontally will never match the number vertically. It's never displayed 5x5 thumbnails - the closest would be 5x8 with an extra row of 2 on the bottom. The same with 6x6 - the closest would be 6x7.