The only way to really determine if the lake is 100% frozen over is with pretty high resolution infrared satellite imagery and that has only been around for 15-20 years, so there really is not enough data to give a complete picture, but there have been a few years when it came very close. In 2003 it got to about 98% covered.
The main problem is not so much it's size, but the fact that it is so deep and the water deeper down remains at a constant temperature of several degrees above freezing, so you always have that water melting the ice. The size does matter too though as the winds will cause any ice on the lake to move around and expose fresh water and also cause water to upwell. So there are a lot of things acting against the water from freezing, so even when it does get a lot of ice on it, that ice does not last for very long.
Whitefish Bay will freeze over in about 65-70% of winter.
-John