Large size resistive micromegas detectors (MM) will be employed for the first time in high-energy physics experiments for the Muon Spectrometer
upgrade of the ATLAS experiment at CERN.
The current innermost stations of the muon endcap system, the Small Wheel, will be upgraded in 2019 to retain the good precision
tracking and trigger capabilities in the high background environment expected with the upcoming luminosity increase of the LHC.
Along with the small-strip Thin Gap Chambers (sTGC) the ``New Small Wheel'' will be equipped with eight layers of MM
detectors arranged in multilayers of two quadruplets, for a total of about 1200 m$^2$ detection planes.
All quadruplets have trapezoidal shapes with surface areas between 2 and 3 m$^2$.
The MM system will provide both trigger and tracking capabilities.
In order to achieve a 15\% transverse momentum resolution for 1 TeV muons, a challenging mechanical precision
is required in the construction for each plane of the assembled modules, with an alignment of the readout elements (the strips)
at the level of 30 $\mu$m along the precision coordinate and 80 $\mu$m perpendicular to the plane.
Each MM plane must achieve a spatial resolution better than 100 $\mu$m independent of the track incidence angle and
operate in an inhomogeneous magnetic field (B < 0.3 T), with a rate capability up to ~15 kHz/cm$^2$.
In May 2016 the first full size prototype (module-0) has been completed and tested at CERN with high momentum pion beam.
The Module-0 construction elements and procedures, and the preliminary results obtained at the test-beam will be presented.