As a normal user
gsettings set org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/ icon-size 24'
and
gsettings get org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/ icon-size 24'
are both fine.
As root:
su - oem -c 'gsettings get org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/ icon-size 24'
is okay, but
su - oem -c 'gsettings set org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/ icon-size 24'
gives no error but does not change the Value