Function points are a standard method frequently used to calculate size and to estimate effort in applications. Although it has been used with good results in the development of industrial software, it is still necessary to improve some aspects such as: the time at which the estimation of effort is performed and the margin of error in the effort estimation. In this article an alternative method for early effort estimation based on use cases is presented. Two new alternative notions of size are used for effort estimation: Transactions and Entity Objects. A case study developed in order to compare the estimations obtained with the two methods for actual projects is described. The result shows that by using number of transactions as a notion of size and the Technique Mean Productivity Value to perform the estimation, the current way of estimating effort can be improved.