Do you think it’s realistic for rats to be dog sized, if humans selectively bred them for size? Or would their skeleton not hold up to such a size? I know there are mini rats now, that are mouse size and have high energy like a baby for their entire life, but I’m not sure a rat enlarged to dog size would work well unless they had thicker bones or more muscle. (By dog sized I mean terrier sized, not like a greyhound or something big)

drferox:

It’s certainly plausible, though I wouldn’t make any guesses about health concerns they may have.

The largest extant rat species on the planet right now can reach chihuahua size.

But extinct rat species were estimated to reach 6kg, which is definitely within the realm of ‘small terrier’.

(image source from link above)

I mean, a capybara is a rodent the size of a medium dog. If you were careful with your selective breeding and picked ones that were still healthy at the larger sizes, I see no reason why you couldn’t get to that big. 

(Which would be terrifying, by the way, since rats can gnaw through concrete)