If you are planning to include a search box in your website or blog, read this:
The Ideal Search Box Is 27-Characters Wide
Jakob Nielsen performed a usability study on the length of search queries in website search boxes. It turns out that most of today’s search boxes are too short. The problem with short boxes is that even though you can type out a long query, only a portion of the text will be visible at a time, making it difficult to review or edit what you’ve typed.
The study found that the average search box is 18-characters wide. The data showed that 27% of queries were too long to fit into it. Extending the box to 27 characters would accommodate 90% of queries. Remember, you can set widths using ems, not just pixels and points. One em is the width and height of one “m” character (using whatever font size a website is set to). So, use this measure to scale the width of the text input field to 27-characters wide.
From: Smashing Magazine