This study develops and tests theoretical formulas for linking country size and party system characteristics. For countries using one-seat electoral districts or nationwide districts, the averages of the largest seat-share, effective number of assembly parties and mean duration of cabinets can be predicted based solely on population. For countries allocating seats by PR in multi-seat districts, the averages of these characteristics can be predicted based on population and district magnitude. We show that first-past-the-post countries of less than one million tend to have highly dominant largest parties and one-and-a-half party assemblies, rather than a balance of two parties. For larger countries, and PR countries of any size, population is not destiny, as far as party system is concerned.