Avalanche

This is the smallest version of the common Shadow ship design, filling the gap between the escorts and capitals.

The reason for it never being seen during the TV show is that the cruiser is, by Shadow standards, too easily defeated by younger race ships and this would destroy the aura of invincibility that the Shadows enjoy. In the episode 'Point of No Return' a Shadow warship is described as a small one, so it isn't too far fetched.