While using youtube I don't know what I pressed, but all video sizes became so small. I cannot understand what is written on the site anymore. Can you please tell me how can I zoom it again?
Is this happening just on YouTube? Or other sites too?
I would try the following things:
Make sure your browser zoom is correct. You can press cmd-0 / ctrl-0 to reset it on most computers I believe. Depending on your mouse, the might be able to scroll through zoom levels too.
To determine whether it's a YouTube problem or a browser problem, I'd try opening YouTube in another browser too. Firefox, IE, Safari, Chrome... There are plenty of options for double checking.