  We discuss finite-size effects in one disordered ${\lambda}{\phi}^{4}$ model
defined in a $d$-dimensional Euclidean space. We consider that the scalar field
satisfies periodic boundary conditions in one dimension and it is coupled with
a quenched random field. In order to obtain the average value of the free
energy of the system we use the replica method. We first discuss finite-size
effects in the one-loop approximation in $d=3$ and $d=4$. We show that in both
cases there is a critical length where the system develop a second-order phase
transition, when the system presents long-range correlations with power-law
decay. Next, we improve the above result studying the gap equation for the
size- dependent squared mass, using the composite field operator method. We
obtain again, that the system present a second order phase transition with
long-range correlation with power-law decay.
