BoyQ, not that I know of, I believe that the size of the picture is theme related, and without adding a new theme to the device I don't think it's possible to increase the size of the picture that displays when someone calls you. Sorry.
It's kind of luck of the draw, and to be honest on a BB7 device I doubt there's much that can be done about the size of the photo that shows. Since there isn't a dedicated theme builder for BB7 yet you may be stuck with pictures the size that they are.
Until BB7 theme Builder is not out, you will be lucky to find solution beside why you wanna see big faces on your screen? :P
Sorry just messing around your phone your decisions