The proper evaluation of the environmental impact is a critical task in the sizing of a microgrid (MG). Generally, this is addressed in the literature by quantifying the generation of emissions in operation exclusively, which tends to underestimate the actual ecological cost of the MG's elements life cycle. This paper proposes an optimal sizing approach that integrates the Life Cycle Assessment (LCA) of the implementation and operation stages by adapting a multi-objective function inspired by the LCA ReCiPe methodology. A sizing case study is presented to discuss the relevance of an adequate estimation of the environmental impact in selecting the MG configuration.