MAORY is the Laser Guide Star (LGS) assisted Multi Conjugated Adaptive Optics (MCAO) module of ESO's ELT where MAORY will provide a corrected field of view of 1 arcminute to its first client instrument MICADO. In the framework of the design phase B, the dimensioning of the LGS Shack-Hartmann Wave Front Sensor (WFS) has been investigated. On an ELT scale, the LGS spot truncation is severe and may lead to a potential loss of performance due to measurement noise as well as non-linearity and truncation induced bias. In this paper, we review the already known design options allowing to cope with LGS spot elongation and truncation problem. Then, we focus on MAORY and the design trade-off of a classical Shack-Hartmann WFS, addressing in particular the choice of the pixel scale and in turn the subaperture field of view. A larger field of view allows mitigating the truncation effects as long as the spot remains properly sampled, by means of extending it. Furthermore, we recall that in the case of a multi LGS WFS system, a large part of the problem can be solved thanks to the redundancy of pupil and metapupil sampling. The key lies in the proper tuning of the wavefront reconstruction, using as priors both the 3D turbulence covariance and well-tuned measurement noise and model errors. The number of reconstructed layers and the definition of their prior altitude and strength have been optimized. The WFS measurement errors are modelled by including the non-uniformity of LGS elongation across the pupil plane. The MCAO performance is evaluated both in terms of pure closed loop residuals and quasi static bias.