Brady Dillsworth , Jul 30, 2003; 08:55 p.m.
The results are slightly better this way than increasing the size to the final output in one shot. Go to Image>Image Size, then increase the length and width by 5-10% until you reach your output size. It will take a while this way, but you can increase your final image to be printed. I don't believe you will be able to reach crisp poster sized images, but they will be acceptable.