  We investigate simultaneous effects of finite system size and global charge
conservation on thermal fluctuations in the vicinity of a critical point. For
that we consider a finite interacting system which exchanges particles with a
finite reservoir (thermostat), comprising a statistical ensemble that is
distinct from the common canonical and grand canonical ensembles. As a
particular example the van der Waals model is used. The global charge
conservation effects strongly influence the cumulants of particle number
distribution when the system size is comparable to that of the reservoir. If
the system size is large enough to capture all the physics associated with the
interactions, the global charge conservation effects can be accurately
described and corrected for analytically, within a recently developed
subensemble acceptance method. The finite size effects start to play a
significant role when the correlation length grows large due to proximity of
the critical point or when the system is small enough to be comparable to an
eigenvolume of an individual particle. We discuss our results in the context of
fluctuation measurements in heavy-ion collisions.
