I like to force small taskbar and also disable grouping.
To disable grouping, the policy is:
User Configuration \ Policies \ Administrative Templates \ Prevent grouping of taskbar items = Enabled
There is no policy to force small taskbar, but there is a registry setting. Therefore we can make a custom ADM file (source: http://social.technet.microsoft.com/Forums/ar/w7itproui/thread/eb25d68d-41cc-4188-a00c-7342992dad05)
reate a file with the extension .ADM mine is Taskbar Extra.ADM on your Domain Controller
Insert this into the file and make sure you save it as .ADM and not .txt