I haven't tested it on the device but it should probably work:
B4X:
obj2.Value = obj1.GetProperty2("Item", 1)
VScrollVisible = obj2.GetProperty("Visible") 'Vertical scroll bar visible
VScrollHeight = obj2.GetProperty("Width") 'Vertical scroll bar width
obj2.SetProperty("Width", 100) '<- sets the width of the vertical scroll bar