Complex systems engineering requires new software tools for virtual prototyping which have to be more relevant in order to meet, at the same time, consumer requirements, standardized rules and market law. These have to be more flexible especially concerning file exchange and reusability. Recently the modelling language Modelica seems to fulfil these needs thanks to its concepts of acausality and multi-disciplinary description. In parallel, the laboratory AMPERE has developed a bond graph-based sizing methodology which, by the use of inverse models, drastically decreases the number of calculus iterations compared to the classical direct approach. The aim of this paper is to highlight the importance of acausality and structural analysis in a design approach and to study to what extent the proposed sizing methodology can be formulated in Modelica. Then first software implementations of the methodology are illustrated by examples processed by the tool MS1 and its Modelica code generator