The size should fit the stallion. Start with normal recommendations, then adjust them as needed to meet his preferences. The novice stallion needs a stimulus to associate the phantom with breeding. The smell of a mare in estrus often starts the process, with the pheromones in mare urine exciting the stallion. The mare doesn't have to be present in the breeding shed, depending on the stallion's preferences.
Many handlers keep mare urine in storage, collecting and freezing portions in small plastic bags.
Our dummy has a lot of scents and smells on it. We lead the young stallion to the dummy and see what his reaction is. I prefer to go with the stallion who shows interest in the dummy. Ewing might introduce thawed urine onto the dummy to encourage the stallion's interest. Both should be confident horse people who treat the stallion with respect and authority.