Normally pretty small, it's believed that wizards have blown them up to a size where they are mortally dangerous. They're as fast and dangerous as spiders. Trust me, they look small, but they'll kill you in a heartbeat!
Besides, what kind of RPG would this be without giant rats? A terrible one, that's what!
Frizzil plans on having giant rats spawnable in every biome, including underwater and in space.
Rat monster spawners, like most other spawners in the game, can be found in towers.
Rats are quite bulky and small. They can pass through 1 block high spaces and it can be annoyingly hard to hit them. Warriors can kill them easily with bash, Mages can kill them easily using a lightning spell, but rangers should have the appropriate tools for fighting them. When fighting them 1 on 1 it's not a big deal. But in groups Defence ( Armor or Dexterity ) is very important. If you are a ranger, you should either shoot explosive arrows at them, and risk damaging yourself in the meantime, or you should always have a club or sword on you. I recommend the latter, just because the sword/club can be very useful at other times. If you are a mage, try to find an open area and run backwards while shooting your best spells at them. If you can't do that (if you're in a jungle biome or other constricted place) I would suggest find some water and going under it and shooting at it from underneath. If you can't do that then I would either advise running, or, if you're a higher level mage than it just use your best spells, and hope you don't die.