  We present a method to measure transverse size and position of an electron or
proton beam, close to the injection point in plasma wakefields, where other
diagnostics are not available. We show that transverse size measurements are in
agreement with values expected from the beam optics with a $< 10\%$
uncertainty. We confirm the deflection of the low-energy 18 MeV electron beam
trajectory by the Earth's magnetic field. This measurement can be used to
correct for this effect and set proper electron bunch injection parameters. The
advanced wakefield experiment at CERN (AWAKE) relies on these measurements for
optimizing electron injection.
