I measure my pieces at their longest and widest places and then add 1/2 to 3/4 inches. If its an outside piece, be sure to add in the seam allowance and still add your 1/2 inch.
Featherweight...I have been using a wallpaper roller for years and love it so much. I use it in all types of quilting. It never never stretches the seam or fabric out of shape. You can sew for ages without using an iron.