Unlike many other breeds of dog that were bred to specific sizes only within recent history, the poodle's three primary sizes -- toy, miniature and standard -- have been around for centuries.
As with many breeds varying size pups turn up in litters and they are only classified differently as you can see here based on their height at the shoulder
http://www.poodle-place.com/poodledescr.htm
No one is sure if the different sizes came from all the same dogs or if there were different breeds that developed in a similar manner
http://www.barkbytes.com/history/poodle.htm
This site from the AKC states that the smaller sizes developed very shortly after the biggest size dog
http://www.akc.org/breeds/poodle/history.cfm
Based on what I've seen in dog breeding for many years its likely that the litters of the original dogs had pups of different sizes and some people chose each different size to breed. The small ones for ladies entertainment and the midsized ones for hunting in different areas than the larger dogs.
Based on early paintings you find poodles starting in the 1400s (15th century) on.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics