First you need to install compizconfig-settings-manager using the following command
sudo apt-get install compizconfig-settings-manager
Default unity launch bar size as follows
You need to open compizconfig-settings-manager by clicking on the Apps Place enter compiz in search box
Once it opens you should see similar to the following screen here click on "ubuntu Unity Plugin"
Once it opens click on "Experimental" tab here you need to change the launcher icon size
After changing the size you should see similar to the following screen