Reduce Application Icon Size Of Gnome Shell

Open Terminal & type

sudo gedit /usr/share/gnome-shell/theme/gnome-shell.css

 
search for:

/* Application Launchers and Grid */ 

You'll see this:

/* Application Launchers and Grid */

.icon-grid {
    spacing: 36px;
    -shell-grid-horizontal-item-size: 118px;
    -shell-grid-vertical-item-size: 118px;
}

.icon-grid .overview-icon {
    icon-size: 96px;
}

Edit the icon-size: 96px to whatever size you like.
Save and
press Alt + F2 button  then type r
or logout and login again to refresh the shell themes.

Enjoy.