The size depends on the device it is running on. It is medium size for desktop and tablet and small size for phone.
sap.m.Size.L
Large size.
sap.m.Size.M
Medium size.
sap.m.Size.Responsive
The width and height of the control are determined by the width and height of the container the control is placed in. Please note: it is decided by the control whether or not sap.m.Size.Responsive is supported.