1. A computer-aided method 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, said one or more variables being dependant on a predefined set of parameters, said method comprising the following steps at least one of which is carried out on a computer processor: a) Defining a parametric space grid setting predetermined distances between its values; b) Obtaining a model for calculating said one or more dimensional variables for whatever point over the parametric space grid through an iterative process with respect to a reduced group of points, of increasing number of points in each iteration, comprising the following sub-steps: bl) 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) Establishing a group with the largest deviation ε between the results provided by the CFD and the ROM models, selecting the e-point thereof and finishing the iterative process if ε is less than a predefined value ε 0 ; b4) Selecting new points over the parametric space grid to be added to the group of points established in step b3) 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 models and going back to sub-step b3).