This client-side mod shows a big version of whatever item you hover over.
Great for builders looking for the right materials!
By default you can toggle it on and off with Shift + Z, which can be changed in the Minecraft controls.
There is also an unmapped keybinding you can use if you want to hold a button down to make it display.
You can change the zoom amount in the config or by setting the hotkeys for Zoom In and Zoom Out.
This mod is originally by mezz and available here, but it broke in recent versions so I fixed it according to his MIT license.
21st January 2021: It does appear that mezz updated his original mod again, but only to 1.16.4. This mod fork here will stay up to prevent mod(pack) rot and for the additional 1.16.5 version that is still not implemented in the original, but I'd say keep an eye on mezz' version and use it over this, once it's updated!