The reducing geometries is very often evoked to explain experimental shifts of water properties occluded inside small cavities. At nanometric level, the deviation of water properties can be modelled and measured and both results agree that from 5 nm (10 nm at max.) to smaller confinement, the finite size effects play a growing role to control the water thermodynamics. In the meantime, collecting large-scale evidences, like the phase transitions and chemical saturations states observed/measured in natural aquifers, the threshold between macroscopic and nanoscopic domains appears much more unclear, extending to 0.1 μm or more. The question thus arises as for the existence of intermediate mesoscopic behaviors, between truly nanoscopic and truly macroscopic domains. Recently, we accumulated a large number of measurements (vibrational and X spectroscopies, phase transitions) on water occluded in different types of cavities (closed cavities in quartz = fluid inclusions, open nano-channels patterned inside silicon wafers, pores membranes, capillaries), and sometimes on the container. These measurements tend to demonstrate that the properties of the water body can change as a function of the distance to interfaces (over 1-3 micrometers !) and as a function of the confinement degree from around 35-40 nm. These measurements open the discussion to the surface-to-volume competition in thermodynamics, especially when addressing two-phases (or more) systems. Thermodynamics of small systems appears attractive to deal with the evident size effect in confinement regime, though it is visible at sizes larger than usually defined. Another challenge is to thermodynamically describe biphasic frontiers using surfacial formalism, possibly requiring, in certain cases, the definition of an interfacial phase with finite width, instead of an abrupt interface between two bulk phases. In both cases, the characteristic threshold between bulk and “others” behaviors should be calculated and generalized on thermodynamic grounds. References Le Caër S., Pin S., Esnouf S., Raffy Q., Renault J.P., Brubach J.B., Greff G. and Roy P. (2011) Phys. Chem. Chem. Phys. 13, 17658-17666. Mercury L., Jamme F., and Dumas P. (2012) Infrared imaging of bulk water and water-solid interfaces under stable and metastable conditions. Phys. Chem. Chem. Phys. 14, 2864-2874. Shmulovich K.I. and Mercury L. (2014) Size effect in metastable water. Petrology, 22(4), 418-428. Bergonzi I., Mercury L, Simon P., Jamme F., and Shmulovich K.I. (submitted) Is in-pores/channel water bulk somewhere? Science. Mercury L., Shmulovich K.I., Simon P., and Bergonzi I. (to be submitted) Negative pressure of liquids: a route to soft fracking. Nature. Bergonzi I., Mercury L., Tas N., Brubach J.-B., and Roy P. (in prep.) Confinement threshold of liquid water and its size-dependent properties from infrared micro-spectroscopy. Phys. Chem. Chem. Phys.