For the high luminosity upgrade of the LHC at CERN, ATLAS is considering the addition of a High Granularity Timing Detector (HGTD) in front of the end cap and forward calorimeters at vertical bar z vertical bar = 3:5 m and covering the region 2:4 < vertical bar eta vertical bar < 4 to help reducing the effect of pile-up. The chosen sensors are arrays of 50 mu m thin Low Gain Avalanche Detectors (LGAD). This paper presents results on single LGAD sensors with a surface area of 1.3 x 1.3 mm(2) and arrays with 2 x 2 pads with a surface area of 2 x 2 mm(2) or 3 x 3 mm(2) each and different implant doses of the p(+) multiplication layer. They are obtained from data collected during a beam test campaign in autumn 2016 with a pion beam of 120 GeV energy at the CERN SPS. In addition to several quantities measured inclusively for each pad, the gain, efficiency and time resolution have been estimated as a function of the position of the incident particle inside the pad by using a beam telescope with a position resolution of few mu m. Different methods to measure the time resolution are compared, yielding consistent results. The sensors with a surface area of 1.3 x 1.3 mm(2) have a time resolution of about 40 ps for a gain of 20 and of about 27 ps for a gain of 50 and fulfil the HGTD requirements. Larger sensors have, as expected, a degraded time resolution. All sensors show very good efficiency and time resolution uniformity.