"I had him from an egg, yeh know. Tiny little thing he was when he hatched. 'Bout the size of a Pekingese..."
-- Rubeus Hagrid (HBP22)
A breed of small dog.
Hagrid said that Aragog was about the size of a Pekingese when he hatched -- and this spider the size of a small dog seemed to Hagrid to be a "tiny little thing." (HBP22)
The largest spider in the world is the goliath birdeater tarantula which can grow to a foot in diameter. That's just a little less than the length of a Pekingese dog, although the dog weighs considerably more.