How much space will be taken by mail app? Does iPad keep only last few mails or it cumulatively use more and more space for mails? Can I set a limit how much max space to allow for mails?

You can decide in 'Settings', 'Mail, Contacts, Calendar' the maximum number of emails that the Mail app will keep on the iPad and whether it will automatically download images, which are likely to be the largest item that will influence the size of stored emails.

You can also decide whether deleted emails are kept in the Trash can or deleted immediately. Clearly all of these factors will influence how much space is consumed by stored mail. However, in reality, the size of stored emails will be small compared to, say, the size of a movie. At most, the Mail app is able to store 200 emails on the iPad and even if these were 1 MB each, that wouldn't amount to more than a fraction of the size of a downloaded movie.

Bottom line - I wouldn't worry about it unless you're really pushed for space.

Tim