Results from the development of a novel sensory and perceptual test to assess the level of altered visual size perception in people with schizophrenia are presented. Here we compare the performances in a visual sensory and perceptual test between a control group and an experimental group of people diagnosed with schizophrenia. We have been using paintings by Salvador Dalí and Rorschach plates to assess images size perception. In this transversal, ex-post-fact and quasi-experimental study we show differences between EG (Experimental Group) and CG (Control Group). Schizophrenics in-patients as compared to controls perceived sizes about 1.3 fold greater than healthy volunteers (p=0.006), thus showing that size perception is altered in schizophrenia. Together with previous results, this particular sensory test for size perception seems to be a useful assessment tool to evaluate the degree and severity of psychotic symptoms in prodromal schizophrenic states, and neuropsychiatric patients in general. It may help prevent acute crisis with proper, and most likely, lower dosages of medication.