GNOME desktop is defined in CSS file  /usr/share/gnome-shell/theme/gnome-shell.css. In order to change the icon size and grid in which icons are organized, open CSS files:

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

and change values in:

.icon-grid {
spacing: 16px;
-shell-grid-item-size: 96px;
}

.icon-grid .overview-icon {
icon-size: 48px;
}