I just checked my tablet and five lines are visible at any one time. However, given that DC is not a text messaging program such as Signal etc. but an IM interface to /e-mail/ I think it is reasonable to allow many more lines to be visible at any one time - dependent on the height of the screen in use, of course. The user should be able to write more than a couple of sentences without needing to scroll to see what he/she just wrote.
Formatting, btw, includes being able to separate an e-mail message into paragraphs with an empty line between paragraphs for clarity. This requires more than five - or six - lines.
My suggestion again is that this entrybox dynamically resizes itself beyond the five lines it does on my tablet up to 2/3 or 3/4 of the height of the screen. This means that the user who writes very short messages still uses a minimally sized entrybox whereas the user who sends longer messages will automatically have much more of the message visible at a single time. The rest of the screen height should still show previously sent/received messages of course.