The easiest way is to save them all in the same file in inkscape. If you select everything and resize it all at the same time, it will resize proportionately. If you have one large outer piece, you might need to either place the smaller pieces inside the larger one or next to it. Make sure that the smaller pieces don't go above or below the big piece, and then resize them all with the size you want the big one to be.
After saying all of that, when you import them into SCAL, your inkscape size becomes more or less irrelevant. If you keep all of the pieces in the same file, you can resize the file and move it on your mat as needed to cut all of the pieces and all of the pieces are the proportional. HTH