  We present the analysis of six XMM-Newton observations of the Anomalous X-ray
Pulsar CXOU J010043.1-721134, the magnetar candidate characterized by the
lowest interstellar absorption. In contrast with all the other magnetar
candidates, its X-ray spectrum cannot be fit by an absorbed power-law plus
blackbody model. The sum of two (absorbed) blackbody components with kT1=0.30
keV and kT2=0.7 keV gives an acceptable fit, and the radii of the corresponding
blackbody emission regions are R1=12.1 km and R2=1.7 km. The former value is
consistent with emission from a large fraction of a neutron star surface and,
given the well known distance of CXOU J010043.1-721134, that is located in the
Small Magellanic Cloud, it provides the most constraining lower limit to a
magnetar radius ever obtained. A more physical model, where resonant cyclotron
scattering in the magnetar magnetosphere is taken into account, has also been
successfully applied to this source.
