In PS7 if I try to paint on a somewhat large (2000x2000 pixel) canvas with a large paint brush (600+) I get a very jagged edge to the paint brush. I've tried everything from trying to raise and lower the resolution of the image, to trying different brushes, and even using different blurs. Nothing helps. Is there any way to get a very smooth edge when I use the paint brush.
Here's an example image (cropped) to show you what I mean by "jagged edge"





