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.