```python
def mean_window(readings: list, size: int) -> list:
    if size == 0:
        raise ValueError("Window size must be greater than 0.")
    
    if len(readings) < size:
        return []
    
    means = []
    for i in range(len(readings) - size + 1):
        window = readings[i:i+size]
        means.append(int(sum(window) / len(window)))
    
    return means
```