As far as the number of lines go it depends on your ability to write well reasoned arguments quickly. I'd expect an essay that'd be at least rated 4.0 would be of this length.
Introduction; always state your position (3-5 lines)
I strongly believe in X [because of Y]. In the following paragraphs I intend to provide well reasoned arguments for the position I've taken.
Three well reasoned arguments (4-5 lines)
Optional conceding to the opponent view (3-5 lines)
While I believe in X I understand that those who believe in Y have some good reasons to do so. For example, ... ,.
Conclusion (2-3 lines) keep it brief.
In my opinion at a minimum an essay should be 20 lines. But a good essay should be targeting 25+ lines. Please remember Quality always trumps quantity when you write an essay. A well reasoned 20 line essay could easily get you a 6.0 while a poorly written 30 line essay could only get you a 4.0 for all your efforts.