  Boundary conditions effects are explored for size-dependent models in thermal
equilibrium. Scalar and fermionic models are used for $D=1+3$ (films), $D=1+2$
(hollow cylinder) and $D=1+1$ (ring). For all models a minimal length is found,
below which no thermally-induced phase transition occurs. Using quasiperiodic
boundary condition controlled by a contour parameter $\theta$ ($\theta=0$ is a
periodic boundary condition and $\theta=1$ is an antiperiodic condition) it
results that the minimal length depends directly on the value of $\theta$. It
is also argued that this parameter can be associated to an Aharonov-Bohm phase.
