What has my 7-year-old done to YouTube?

[Wally_The_Rabbit]

My child has somehow changed the setting on YouTube and now all text, images and the screenbox itself have all completely shrunk. The text is so small that it's illegible.
What has he done and how can I restore the standard settings?

 

[beers]

Either hold down CTRL and spin your mouse wheel away from you, or click this guy.

 

[Wally_The_Rabbit]

Brilliant!!!! Thank you very much for your help! Much appreciated