I'm using ubuntu 14.04 & I need to decrease all the icons size in one move, can I do it or I must resize each icon?
Right-click on your desktop, click on Desktop Settings. There you should have 3 tabs: Background, Menus, and Icons. CLick on Icons and you should see Icon size. Adjust Icon size accordingly.
The comment above gives the correct answer:
From this answer: askubuntu.com/questions/129820/…. -- Open file explorer -- Edit > Preferences > Icon View Defaults Set as you wish. – md_essam
For the file manager in Ubuntu 16.04 you use scroll wheel and crtl in the file manager, only it does not get any smaller than 32 pt.