How can i set the fontsize of different document elements?

I am using \documentclass{article} to write an article but I should change its default font size to other standards. How can I change the font size of each title or body underneath of each title? How about some words in the document?

How can I understand that each of commands \small, \Huge, etc is equivalent of what font size?

This what I want to have: I have written the font size in front of each section, also I want to left justify the abstract section.