If I remember correctly it was made up of roughly 200 Slave rigged Dreadnoughts. These dreadnoughts had crews of 2,000 instead of something like 16,000. About ten years before the clone wars(according to dark force rising) it was lost. The story is that the crewers were infected with a hive virus and the crew of the Katana made a blind jump into deep space and brought the rest of the fleet with it to protect other planets. It was explained that back then there was no cure, but it can be cured with Bacta which wasn't around then.