How to start/open a file/folder that contains space in its name through command-line?

You would use:

start "" "c:\program files\"

That is because the first parameter is used as the title of the window, and is oddly enough, enclosed in double quotes.

Edit:

Here is a source about it: SS64

As an example, if you just type start "title" it opens a new cmd window with the title "title" in the title bar.

Do you specifically need to use start?

You can use explorer "c:\program files" to give you the effect you are looking for.