In his 1789 speech against the abolition of slavery, William Wilberforce aimed to have a significant impact on his audience by fostering a deep understanding and consideration for the magnitude of the topic he was discussing. Therefore, the most fitting interpretation of his rhetorical intent is:
He hopes they consider the magnitude of the topic he is discussing.
Wilberforce employed powerful moral appeals and vivid imagery to underscore the devastation and inhumanity of slavery, urging his audience to recognize the ethical implications of their actions and beliefs regarding slavery and to inspire them to take action towards abolition.