I’m glad you’re looking into the industry standards for manuscript formatting. This is one important way that you show editors and agents you are serious and professional about your work.
While you certainly want to present a manuscript that’s pleasing to the eye, don’t agonize over white space and number of words on a line. You’re probably used to seeing fiction printed in published books, so a manuscript for submission will look much more open and light. But those qualities are exactly what agents and editors are expecting. They read manuscripts all day long. They may even make notations or edits as they read. As a result, it’s important to keep the font large and readable to make this process easier.
If a publisher or agent requests a certain font, use it. (And some do have a strong preference.) If not, choose from the fonts that fit the criteria—Courier New and Times New Roman are common choices—and stick with the standard 12 point size.