As part of the Hi-Luminosity LHC project, a set of Nb3Sn quadrupoles are being developed aiming to enhance the performance of the inner triplets. The new magnets, identified as MQXFA and MQXFB, will share the same cross section with two different lengths, respectively 4.2 m and 7.2 m. During the magnet development, three short models were tested, along with a number of mechanical models, demonstrating the capability of the magnet cross-section to achieve the desired performances. Same performances are now required for the full-length magnets. To insure this, the authors studied the impact of the magnet length on the capability of the structure to provide an adequate support to the coils. FE and simplified analytical models were used to evaluate the impact of the magnet length on the stresses in the magnet ends and coil elongation during powering. The models were calibrated using the results from the short models tests, and used to provide an indication on the required prestress and its foreseen impact on the magnet performance.