A group of turtles is called a "bale".
A group of turtles is also called a "dole".
A group of female turtles or hatchlings is called a "nest".
A group of tortoises is called a "creep".
The largest species of turtle ever was called Archelon. (Actually, this is kind of a misnomer, since many would consider Archelon a stem-turtle, not an actual turtle species. If you agree with this reclassification, then the actual largest turtle species ever is Stupendemys geographicus.)
The largest living species is the Leatherback Sea Turtle, Dermochelys coriacea.
The smallest species is actually a very contested topic. As of right now, the general consensus is that it is the Vallarta Mud Turtle, Kinosternon vogti.
Less of a fun fact, but, as of now, the rarest animal in the world is the Yangtze Giant Softshelled Turtle, Rafetus swinhoei, which only has 3 known extant specimens.
The Siamese Narrow-headed Softshell Turtle, Chitra chitra chitra, is the largest freshwater turtle.
More coming soon...