The classic answers are that a government should protect its citizens from attack (the military, the police) and should act as a neutral ajudicator in contractual disputes (the courts) and that's it.
What about "the poor"? The classic answer is "you will not be prevented from helping them".
It's those 'classic answers' that are the important part of Libertarianism. If you get rid of them then Libertarianism just becomes 'The government should be the size required to do whatever it should do, and be neither bigger nor smaller', which would be completely vacuous.
What about "the poor"? The classic answer is "you will not be prevented from helping them".