backalley, regarding file size, in this particular application it's analogous to film speed: you can make a good 8x10 with ISO400, but a better 8x10 with ISO100. So with 12.2mb you can make a good 8x10, but with 20mb you can make a better 8x10.
The other advice on resizing the image is right. Most pro labs crop to fit, so in Photoshop if you make your canvas size (Image: Canvas Size) the same as the final print size, you can resize the image on the screen to fit the canvas.
Alternately, if you don't want borders, you can crop the image yourself to the print dimensions.
Rob