1. A computer-aided method suitable for assisting in the design of an aircraft by providing the values of one or more dimensional variables for the complete aircraft or an aircraft component, being said one or more variables dependant of a predefined set of parameters, characterized by comprising the following steps: a) Defining a parametric space grid setting predetermined distances between its values; b) Obtaining a suitable model for calculating said one or more dimensional variables for whatever point over the parametric space through an iterative process with respect to a reduced group of points, of increasing number of members in each iteration, comprising the following sub-steps: b1) Calculating the values of said one or more dimensional variables for an initial group of points using a CFD model; b2) Obtaining an initial ROM model from said CFD computations and calculating the values of said one or more dimensional variables for said initial group of points using the initial ROM model; b3) Selecting the e-point of the group with the largest deviation ε between the results provided by the CFD and the ROM models and finishing the iterative process if ε is lesser than a predefined value ε 0 ; b4) Selecting new points over the parametric space to be added to the group of points as those points placed inside the parametric space grid at a predefined distance from said e-point; b5) Calculating the values of said one or more dimensional variables for the new points using the CFD and the ROM model and going back to sub-step b3).