Field size

How do I use Javascript/HTML to ensure the user doesnot enter more than 3 characters in a form field?
When we visit some websites, the form doesnot allow you to enter say the username- for more than 4 characters long.The cursor doesnot move forward.
How do we do that?

Lucky, judging from the types of questions you are asking, you really need to get ahold of a basic HTML reference which could answer a lot of your questions.
One is available here. And if you look up the input tag, you will find that the maxlength attribute does exactly what you want.