My take on the Breen ships is that they are very poor in combat - Defiant was able to finish off two of them easily in the second Chintaka battle with just a couple of hits each, so their shields and hull can't be too strong. Their offensive weaponry also seems fairly poor - it took several hits with their torpedoes to destroy the Defiant, even though the ship was unprotected by either shields or the structural integrity field at the time. I've assigned them strength numbers which are fairly low in these categories. In order not to make them too weak I've boosted the agility and speed to fairly high levels, but overall they still score quite low.

The size of the ships is hard to judge - most of the scenes I've seen them in with other ships makes them look about 20% longer than the Defiant, but there is at least one scene in the episode of the Defiant flying past background wreckage which makes the Breen ship look huge, in the 500 metre plus range at least. For now I'm sticking with the smaller size, but would be happy to hear from anybody who knows differently.