Today I decided to clean up my hard drive and have deleted many unused stuffs. One thing I found out is that it's very useful to display the size of files underneath the files. It helps you know which files are occupying more hard drive space. Here is how to make Caja file manager display the file size under each file.
Open the file manager, then click on Edit > Preferences:
You will have the File Manager Preferences window open, on which select the Display tab:
You can see that the first option of Icon Caption is None. Click on it and change it into Size :
And now you can see that Caja is displaying the file size under the each file: