  The precession of electron spins in a perpendicular magnetic field, the so
called Hanle effect, provides an unique insight into spin properties of a
non-magnetic material. In practice, the spin signal is fitted to the analytic
solution of the spin Bloch equation, which accounts for diffusion, relaxation
and precession effects on spin. The analytic formula, however, is derived for
an infinite length of the 1D spin channel. This is usually not satisfied in the
real devices. The finite size of the channel length $l_{\text{dev}}$ leads to
confinement of spins and increase of spin accumulation. Moreover, reflection of
spins from the channel ends leads to spin interference, altering the
characteristic precession lineshape. In this work we study the influence of
finite $l_{\text{dev}}$ on the Hanle lineshape and show when it can lead to a
two-fold discrepancy in the extracted spin coefficients. We propose the
extension of the Hanle analytic formula to include the geometrical aspects of
the real device and get an excellent agreement with a finite-element model of
spin precession, where this geometry is explicitly set. We also demonstrate
that in the limit of a channel length shorter than the spin relaxation length
$\lambda_{s}$, the spin diffusion is negligible and a 0D spin transport
description, with Lorentzian precession dependence applies. We provide a
universal criterion for which transport description, 0D or 1D, to apply
depending on the ratio $l_{\text{dev}}/\lambda_{s}$ and the corresponding
accuracy of such a choice.
