Is it possible to tell a dog gender based solely on its size/head/face

3. The Backs of Female Dogs Are Longer

There are several dog breeds where the appearance of the back allows females to be identified as males. Certain breed standards occasionally require female dogs to have longer backs. Given that longer bodies in females may create the appearance that these dogs are better suited to carry offspring, this may be something that is expected in female dogs, as explained by Caroline Coile in the book “The Chihuahua Handbook.”