Which is the biggest house cat?

Owlwings is correct, but the Maine cat is probably related to the other larger breed of cat, the Norwegian Forest cat. I believe that the only "domestic" cat that has real wild blood is the crossbreed Savannah's. That's not to say that there hasn't been natural crosses with the European wild cat, the Desert wild cat or the Jungle cat. It is suspected that the Egyptians might've "created" the domestic cat when they put Jungle cats and Desert cats in a "temple" together, though I understand that most of the genetic markers for the domestic cat actually come from the Desert cat. The fact that the Desert cats are aggressive while the Jungle cats are less so, does indicate that there was probably some cross breeding going on. However, because the tamer cats would stay with the humans and the wild one wouldn't, it maybe be that they sort of "selectively bred" themselves domesticated.

It rather depends on how you define "large". Some cats of normal size can be very overweight and could therefore be called 'large' but if one takes size of frame or length into account then the story is rather different. The Maine C00n is generally recognised to be the largest breed of house cat, though there are larger domestic pets, sometimes called Savannah, which are crosses between house cats and the South American Serval. The Serval is unique in that it naturally defecates in running water and so can be quite easily trained to use a human toilet. Presumably some of the domestic cat/Serval crosses can also be trained this way. There is also a fairly new breed called Siberian, which is a large cat, but I can't find much detail about it. The Maine C00n, itself, is said to have some wild cat blood in its genealogy. The Maine C00n is stated to average about 25lbs, though many are larger. The Birman is another breed which can be very large (though it is generally classed as a "medium" sized cat). There have been several records of cats weighing 36 to 40 lbs and of domestic cats which can easily put their paws on a kitchen work surface when standing. This approaches the size of a medium to large dog, such as a Labrador or Setter. I think that all such breeds must have some history of breeding with wild cats, though they are clearly tame enough to be considered domestic cats. (Auntie MyLot thinks that the name of the Maine variety is an 'offensive word'!)

I love cats, any type of breed..i like them all.