This extension gives you the ability to zoom in and out in Slither.io.
Slither.io does not have an innate zooming function but there is one confirmed way to be able to zoom in and out in-game, provided you are playing at a PC and using Chrome.
Slither Plus is an extension that adds a number of features to Slither.io to make the game more enjoyable, and unlike competitor Slither.io Mods V3 it hasn't been removed from the Chrome Web Store for being malicious.
Though Slither Plus has a number of features (including an even lower graphical mode for low-end computers to play without lag) the zoom feature is one of the most sought-after--though zooming out to have a larger view than other players is very much considered cheating.
If you're all right with cheating in Slither.io (I'm not really, but Slither Plus has some nice extra features) you can grab it up on the Chrome Web Store for free. The website also has a separate script download for Opera and Safari users, but it's better safe than sorry when downloading unofficial fan-made scripts. Stick to the Chrome version because it is confirmed to be safe.
After you have installed the extension and go to the Slither.io website you will see a few options under the Nickname field, one of which being Zoom.
Make sure it is toggled to 'ON', input your nickname, and press 'Play'.
Once you're in, you can zoom in and out using the mouse scroll wheel. It's as easy as that!
We here at GameSkinny don't like to condone cheating but there's no stopping you if you want to, and it's best you keep your computer safe in the process. Be wary of scripts and extensions that are not able to be hosted on the Chrome Web Store as that implies they are not safe--it's not worth risking your personal information just to cheat at a free to play game.