If you're printing fairly large, you'll probably need more pixels in your final image. Therefore you need to resize your image through "resampling" to get more pixels.
In Photoshop, after choosing Image>Image Size, click on "resample" and "constrain proportions". If the firm you are using to make your prints suggests 300 ppi, then enter that value. Then enter one of the dimensions you are looking for (L or W) and the other will be shown. When you click OK, your image will be resampled to those new pizel sizes, and the final dimensions shown at 300 ppi.
Brad
