Rainwater harvesting is an ancient practice aiming to cover water needs for domestic, irrigation and livestock uses. In this study, the rainwater harvesting tank size was investigated to meet five water-need levels of a mixed goat–sheep farm using a daily water balance method. This method was applied using daily rainfall data for a period of 16 years from six meteorological stations in selected regions of Greece, characterized by different rainfall regimes and well-developed livestock activity, taking into account, among other parameters, the water needs of animals, the rainwater collection area and the runoff coefficient. There is a great variation in the rainwater harvesting tank size among the stations studied due to differences in the annual rainfall and the maximum dry period. Results showed that meeting full demands (100% reliability) requires tank sizes ranging from 20 m3 for short dry period stations–low demand scenario (320 L/day) to 115 m3 for long dry period stations–high demand scenario (576 L/day), assuming a maximum collection area of 450 m2. Correspondingly, reliability analysis showed that very high values of reliability (95%) can be obtained with tank sizes ranging from 10 to 85 m3, respectively.