Very small, like 10-15 rooms.

Those rooms are full of generally tough agro mobs. There are a lot of interesting things they do you don’t often see. Like drive, mirror image, and some unique skills PCs don’t get.

They also track rapidly.

Some of them even care about other mobs in the area and that can affect their behavior in ways that are unique to the lairs.

They are pretty challenging. Huge groups can steam roll them (well even huge groups face big risks in at least 2). But well coordinated groups of 2-4 can crack a lot of them.

I’m not sure if it’s in the helpfile but when they were launched it was indicated that there are advantages to killing all 5 ancient dragons before going after Tiamat.