Botnet size is one of the most important characteristics to evaluate the threat of botnet. Previous studies of botnet size basically focus on details and concreteness instead of nature of problem due to polymorphic, versatile, complex behavior of botnet and partial observation on network. This paper investigates the nature of botnet size, upon which four issues are introduced. The paper summarizes the existing solutions to the four issues and analyzes the challenges to resolve them deeply. Finally, some valuable research works of next step are proposed.