Schizophrenia affects not only mental function but also movement. We compared the movement of patients with mild schizophrenia and healthy control participants during a bimanual assembly task and examined whether changes in object size affected unimanual and bimanual movements. Fifteen patients with schizophrenia and 15 age- and gender-matched control participants were instructed to bimanually reach for and assemble objects. We manipulated the object size for the left hand (large vs. small) and measured movement time, peak velocity, and bimanual synchronization to represent movement speed, forcefulness, and bimanual coordination. Patients with schizophrenia showed slower and less forceful unimanual movements and less coordinated bimanual movements than control participants. Increasing the object size elicited faster and more forceful unimanual movements and more coordinated bimanual movements in patients. The results suggest the need for movement rehabilitation in patients with schizophrenia and the possibility of manipulating object size to optimize patients’ movements. These results benefit the practice of evidence-based therapy.