"Два литра молока" - in this case, the form of the word "литр" because the plural form. One - "литр", two - "литра", three - "литра", etc.
"бутылку" and "кусок" - is the accusative case. In the Russian language most often checked by substituting a question (in this case, the accusative case) "Вижу что?" - "бутылку", "кусок". If the form is the same, it means that it is the accusative case.