What is the smallest mammal pet?

I would think there are some rodents smaller than a mouse that could be kept as pets. I know in the wild there is a species of pygmy shrew. There are also many different mouse species and mouse breeds. The European wood mouse, for example, is smaller than a house mouse, but it is not really a suitable species to keep as a pet. A wood mouse is a wild creature that is very nervous and jumpy. I nursed an injured wood mouse for a few days. It leaped out of its box and ran off as soon as it was better.