Answer:
One Page and ask not more than twenty questions.
Step-by-step explanation:
Direct-mail questionnaire should not be more than one page because the response rate of mail questionnaire is very less as respondents are having no incentive for taking the questionnaire.
Also most of the times mail questionnaire land up in Spam box of respondent and ignore it. Mail questionnaire questions can be interpreted differently by different individuals and may lead to ambiguity in responses.
Respondents also can’t ask any doubts in mail questionnaire and generally leave the questionnaire partially filled. Such partially filled questionnaire and low response rates of mail questionnaire does not give any concrete response of targeted individuals on the topic under consideration and so mail questionnaire has to be coupled with other research techniques.