For folks thinking a 3 is too big and a 0 is too small the new Pi 3a might serve.
This unit might fall into a dark hole between the two. It has the same RAM as the Zero, better WiFi and a more powerful processor. For Pi-Hole, the A+ would be a bit nicer to work with due to the faster CPU, but DNS performance will be similar.
The big advantages of the 3B+ for Pi-Hole are built-in ethernet and 1 GB RAM, both useful for the Pi-Hole application. For $10 more than the A+, you get those and three additional USB ports.