
CandyFace
The preview window, shows its current scale right beside the window title: 0.333x 0.500x, 1x, 2x etc... If you zoom in or out so its 1x, then that is the actual preview size. The border changes depending on how much you have zoomed in the canvas. You can't hide/remove the red border, but you can zoom out on the canvas, making the red border get out of sight, and then use the preview window to zoom in at 1x.
I hope that made sense :)