We have a small door for our Yorkie and Pom. Our Yorkie had no trouble with the door as soon as we got her (as an adult.) But, our Pomeranian didn't like the original hard plastic spring loaded door that came with it. We had been propping the door open so she could use it but we did get a mouse in the house once. So my husband changed the door to a flexible piece of plastic like they use on walk-in freezers at restaurants and that has worked great! No mice !!
Also, where we put our doggie door was not right out to the same level as the ground so my husband built the girls a "deck" right outside the door with steps to the ground. They love their deck and laze around in the sun on it as much as possible here in Seattle!
