  We investigate the process of dimensional reduction of one spatial dimension
in a thermal scalar field model defined in $D$ dimensions (inverse temperature
and $D-1$ spatial dimensions). We obtain that a thermal model in $D$ dimensions
with one of the spatial dimensions having a finite size $L$ is related to the
finite temperature model with just $D-1$ spatial dimensions and no finite size.
Our results are obtained for one-loop calculations and for any dimension $D$.
For example, in $D=4$ we have a relationship between a thin film with thickness
$L$ at finite temperature and a surface at finite temperature. We show that,
although a strict dimensional reduction is not allowed, it is possible to
define a valid prescription for this procedure.
