The Size-map editor is an annexe of the GIMPressionist filter. You can get to it by clicking on the Edit button in the ‘Size’ tab. With this editor, you can set the size that brush strokes given by filter will have. You can place one or several vectors. You can set their strength. They will act on the corresponding area of the image. Si possono posizionare uno o più vettori. Per ciascuno di essi si può impostare l’intensità. Ogni vettore agisce sull’area corrispondente dell’immagine. In this window you can place your vectors. By clicking on the Add button, you add a vector at the center of the window, whereas clicking with the mouse Middle Button puts it where you click. Vectors are red when selected, and gray when they are not, with a white point at tip. With the scroll bar on the right of the Vectors panel, you can set the image brightness. This can be very useful if the image is very dark/bright and you can’t see vectors well. This Preview gives you an idea of the action of the different vectors. The size of squares represent the size of the brushes and their strength. This slider acts on the influence domain of the selected vector. This influence lowers with distance. Source.