The opinion can be found here in all official languages of the EU.
Just enter "Devolution in the European Union and the place for local and regional self-government in EU policy making and delivery" for the title.
As Romania is planning an administrative reorganization, the following paragraph is relevant:
71. Calls on the Member States to carry out any territorial reorganisation or future local government reforms in a careful, considered and well-planned way and in accordance with the Charter on Local Self-Government and the European Reference Framework for Regional Democracy, which guarantees and strengthens local self-government based on the principles of democracy, proximity and decentralisation;