15 years ago the UN participated in the definition of the Lleida Statement on Intermediate Cities and World Urbanisation (available at http://www.unesco.org/most/ciudades.pdf), focused on local autonomy, international cooperation between intermediate cities, and urban planning and management. There is a strong focus on developing world cities, an issue that has clearly evolved since then.
The French Federation of Mid-sized cities
(http://www.villesmoyennes.asso.fr/). It groups some 200 Mayors of mid size cities or inter-municipal managers. The complexity of the administrative map of local government in France and the intermunicipal cooperation schemes lead to a great relevance in their work for fiscal and partnership schemes, but they also work in other things.
In the US there was a peak of interest on mid-sized cities some 10 years ago, as the Rochester Conversation on Mid-sized Cities shows (available at http://livable.org/storage/documents/reports/Other/The_Mid-Sized_City_Exploring_its_Unique_Place_in_Urban_Policy.pdf)
In Latin America the works of the CEPAL and other entities show that mid-sized cities are those with the higher growth ratios, and innovative places.