How does the visual size of a text input field affect users' expectation of how much they should write?

Question

It's well-known that you should size your input fields to the amount of text you want the user to write. There's an example of that on this very page: even though the "Your answer" field can stretch to fit any size of text, the default size is about a dozen lines, to discourage users from writing one-line answers.

I'd like to know more details on the relationship between input field size and user behaviour, if that relationship is generally known. Put simply, how big should the field be for a given desired input length? Of course, writing more or less text isn't the only possible outcome of putting a larger or smaller field: users might just walk away (thinking the form's not for them). How big a problem is that?

Most importantly, how much does your answer depend on the type of input? Is the relationship the same for feedback forms, posts to fora/communities, comments on other users' content, &c.? I'd assume it depends on language too, but I'm only interested in English.

Answer 1

One example of this is Jakob Nielsen's research which suggests that The Ideal Search Box Is 27-Characters Wide...

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.

I would also recommend further investigation of Jakob Nielsen's usability research to help you find your answer.

Answer 2

There may not be a universal rule that can be applied, since the reaction of a user to the text input size will relate to the task they wish to accomplish, and their expectations of the application / site.

Usability testing with your target users is probably the only way to accurately guage their particular expectations.