Many companies and governments perform information technology (IT) projections repeatedly for higher competitiveness and efficiency. Moreover, as the sizes of IT projects increase, the manpower and cost estimation of the projects are getting more important. In this research, we present a size estimation method of information system development projects on the basis of the function point method at the stage of information strategy planning(ISP), and illustrate the size calculation of an example IT project. According to the method, we first identify the types of projects, the scope and boundaries of size estimation, then count data and transaction functions from the artifacts of the ISP project. The unadjusted function points are adjusted to function point by the table of the Early Function Point. The way of calculating the sizes of IT projects will support successful IT projects by estimating reasonable manpower and cost for the projects.