  We analyze the Kondo effect of a magnetic impurity attached to an ultrasmall
metallic wire using the density matrix renormalization group. The spatial spin
correlation function and the impurity spectral density are computed for system
sizes of up to L=511 sites, covering the crossover from $L<\ell_K$ to $L >
\ell_K$, with $\ell_K$ the spin screening length. %Strong mesoscopic variations
of the Kondo temperature $T_K$ %and of the spectral features override, to some
extent, the %even/odd effect predicted earlier for averaged quantities. We
establish a proportionality between the weight of the Kondo resonance and
$\ell_K$ as function of $L$. This suggests a spectroscopic way of detecting the
Kondo cloud.
