This porn video tag is related to a specific category in the adult industry. "Shemale" refers to transgender individuals with male-to-female characteristics, who may have both male and female genitalia. In this context, it's used to describe a performer or scene involving such an individual.
"Small dick" is self-explanatory; it indicates that the performer in question has a smaller than average penis size. This tag can be applied to either cisgender male or shemale performers.
So, this tag refers to a specific type of porn video involving a shemale performer with a smaller penis size.