EF 300 2.8 II - airtight plastic bag

The manual recommends using an airtight plastic bag when going from a cold place to a warm and vice versa. A few questions:
1. Do you practice that and does it really do the trick?
2. What size bag will be needed for the 300 2.8 ii?

1: as long as you can keep the warm air from circulating around the lens freely you are fine. For instance, your regular camera bag is fine so long as you don't open it before the lens is at room temp. The singular plastic bag will allow the lens to warm up quicker.

2: A big one.

I have always just left the camera and lenses to warm up in the backpack to warm up. So long as there is no air circulating around you gear there should be no problems.