It would be pretty much impossible for feral dogs to establish more than one breeding population. Within two generations there wouldn’t be any more giant dogs or toy dogs. First because those types of dogs would be at a severe survival disadvantage compared to average dogs, and second because the ones that did survive would mate with regular dogs and not solely with other giant/tiny dogs.
A chihuahua CAN mate with a great dane, if the chihuahua is the male. The only way we’d see speciation is if there is reproductive isolation. That cannot happen with feral dogs unless there is a physical barrier like dogs on an island. A surviving great dane might not mate with a surviving chihuahua, but the great dane is going to mate with the lab, and the chihuahua is going to mate with the terrier, and the lab/dane is going to mate with the chihuahua/terrier. It isn’t like all the great danes live on one island and all the chihuahuas live on another island and 10,000 years later they can’t interbreed anymore. Dog breeds are thoroughly mixed geographically, and don’t have any behavioral or social barriers to interbreeding.
There are enough behavioral and social differences between dogs and wolves that it is possible that dogs and wolves wouldn’t merge back together into one breeding population.