A key aspect of designing open-ended questions that is sometimes ignored is the comment box that is presented below the question. The size of the box guides respondents' expectations as to how long their answer should be. Small boxes suggest that only brief answers are sought which could mean missing out on respondents' full thoughts. Boxes that are too big can put respondents off as it can look like they are expected to write an essay.
The image below shows the default size of the comment box for an open-ended question in SurveyMonkey:
For a typical open-ended question I would make this box around twice as big. To do this in SurveyMonkey click the "Edit Question" button on the "Edit Survey" page. The window shown below should then appear:
Check the "Question Size and Placement" box and you will be able to change the size of the comment box. The default size is 3 lines deep by 50 characters wide. I tend to change the number of lines to 5 or 6 but keep the width at 50 characters. However, this is largely due to personal taste rather than any hard and fast principles so feel free to experiment with different shapes of comment box. Just remember not to make the box too large or too small.