I use GNOME instead of Unity.
I especially love the overview available in gnome. But I find the icon sizes to be too large. If you want to edit the size of icons in the applications tab then you have to edit the gnome-shell.css file. This was just a google search away. But if you want to change the size of icons in dock then google doesn't have the answer. You can't change it by editing the css file. This is because the size of the dock changes dynamically as and when applications are added or deleted. So:
To change the size of icons in application launcher/dock on the left in overview mode, you need to edit the js that is handling this.
sudo gedit /usr/share/gnome-shell/js/ui/dash.js
Edit the line
let iconSizes = [ 16, 22, 24, 32, 48]; //Original line: iconSizes = [ 16, 22, 24, 32, 48, 64 ];
Gnome 3.6 has an even better overview mode. link