A pocket dreadnought was a military space craft. Probably "pocket" meant that it was very much at the small end of the dreadnought family. The only named pocket dreadnought in the Vorkosigan Saga was the Triumph; it had a crew of 60, very good acceleration, and quite a bit of armament.
The only book with a description of the capabilities of a pocket dreadnought is:
- The Warrior's Apprentice chapters 11,12