Hello
I would like to make poster presentation in 90cmX140cm. I have doubt that libre office has default scale units are cm or Inches?
For poster size, set dimensions in Format>Page. Use a specific unit after the number. You have many choices: cm, in, pt or even m, mm, ft, …
To constrain the rulers to a specific unit, right-click on it and choose from the pop-up menu.
If this answer helped you, please accept it by clicking the check mark 
to the left and, karma permitting, upvote it. If this resolves your problem, close the question, that will help other people with the same question.