do i really need to write pages long answer??

i am a commerce student and my question is when im writting answers for economics,english,commerce i am facing difficulty to strech answers and i have seen some students write a page long answers vs some students writing 6-7 lines max . are short answers valid ?? or i have to write an entire essay for them also tell me how many lines should i write according to the marks like any question of 2 or 4 marks