For the last 15 years, a methodology has been developed for sizing mechatronic systems. Based on the use of inverse models described by bond graph, it has in particular the advantage of drastically decreasing the number of calculus iterations, compared to the trial and error procedure of the classical direct approach. The aim of this article is to extend this sizing methodology to the case where only a part of the specifications can be translated in terms of functions of time (and so where only a part of the model can be inverted) and where the other part can be formulated as an optimal control problem. In particular, the illustration of this extension on an academic example of two masses in series will show how a partially inverted model can be coupled with the graphical construction of an optimizing bond graph established in recent articles.