#You can edit /usr/share/gnome-shell/theme/gnome-shell.css.

#Don't forget to make a backup of the original file!

#Search for a line starting with #dash {.

#In the following lines you can change the properties like .icon-grid { spacing and icon-grid .overview { icon-size.

#After your changes you have to restart the Gnome Shell with Alt+F2 and typing r and Enter.