I am not tall enough to become the function well, nor lean enough to be thought a good student; but to be said an honest man and a good housekeeper goes as fairly as to say a careful man and a great scholar. The Clown is going to pretend to be Sir Topas the curate. Ideally, a curate would visit the sick of his parish and provide counsel and comfort. In this case, the sick man is Malvolio, who is (thought to be) insane. The curate who provides such a service should be "a careful man" (one who takes care to do his duty) and "a great scholar" (one whose studies will provide him with an understanding of Malvolio's affliction). Of course, the Clown is not a curate: he doesn't look like one, and doesn't have the knowledge that a curate should have. However, he reasons that it's as important to be known as "an honest man and a good housekeeper," as to be known as a good curate. Ironically, the Clown, though he is "honest" (loyal and trustworthy) in his fool's way, he is not a "housekeeper" (manager and host) at all.