Photoshop CS5: Resize text box by percent of canvas size?

I am making an action to batch add my copyright to many photos. The problem is I've cropped many of them and even made some into a collage. When I play my action it maintains the same font size for each photo (which makes sense). Since they have been cropped the text is a different size for each photo as seen visually on the screen (since the pixel ratio changed but the size ratio stayed the same). I will be posting the pictures online and want them to all have the same size copyright as seen on screen. Not sure if I need to change the canvas ratio somehow to force that (but don't want to loose quality or pixels) or if I can set the text box to a percentage of the canvas size or what?? Any suggestions are much appreciated!