This short paper presents our initial results in the study of the relationship between size, effort, duration and number of contributors in Free/Libre Open Source Software (FLOSS) projects. We have focused in projects containing more than 600 KLOC thousand lines of code, and stored in repositories compatible with data extraction tools that we are familiar with. Software cost estimation has been an active area of research for almost 30 years. However, to our knowledge, no previous similar research has been conducted in large FLOSS systems. Our research question is how much effort, on average is required to create a large FLOSS system. The quantiﬁcation of such relationship is important to help, for example, planning the evolution of future FLOSS projects and in comparing them with proprietary systems. It can also help to identify the productivity 'baseline' to which compare any future improvements in process, methods and tools for software evolution.