Observational constraints on star cluster formation theory - I. The mass-radius relation S. Pfalzner, H. Kirk, A. Sills, J. S. Urquhart, J. Kauffmann, M. A. Kuhn, A. Bhandare and K. M. Menten A&A, 586 (2016) A68 Published online: 27 January 2016 DOI: 10.1051/0004-6361/201527449