Another option. Let's say none of the print size choices fit the ratio you really want. For example, you've shot a panoramic image that is like 8 inches tall and 24 inches wide. Create a new layer and fill it with white (use the paint bucket--be sure the white square is the foreground color in the two squares near the bottom of the tool bar. Use the little arrow to toggle back and forth between white and black and background and foreground) .
Move that layer to the bottom, so it is underneath your picture. Now, go to "image" and select Canvas size. Since 8 x 24 isn't a standard print size, pick a size that is. Say 15 x 24.
Now, both layers will be over a background (most likely transparent) that is the full 15 x 24 size. Select your white layer and resize it. (edit menu - transform-scale) to fill the full 15 x 24 size. Now, select the layer that has your picture on it and do any minor adjustments on it that might be needed to get it to fit within the 15 x 24 canvas. (use the same edit menu- transform-scale to resize your picture, holding down the shift key so you don't change the proportions of the image.
Basically, you'll have your picture on a white background with the background matching a standard picture size and your actual image the size you want it. Save a jpeg (which will flatten the image) and then you can order a print in the standard size and it will end up as a print with a white background. You'll have to matte it of course to fit your image, but this is a cheap way to get a custom print size from printers.
I would never recommend doing that for photos, graphics or text, coloured layers fine, but not for photos. Transform-scale is destructive, it will also cause pixelation very quickly.
If you want to make a 15x24 with a fill layer to prevent printer auto cropping there is a better way, isn't there always!
Open your image, go Image-Image Size, check the Constrain Proportions and the Resample Image boxes, put in the resolution you want, 240 dpi is fine for prints, then put in the side that fits, for a 3:2 image you'd put the 15" side in and that would give you a 15x22.5 printed area. Click OK.
Then go to the canvas size and increase that to 15x24 press OK, job done. No need for layers or transforming.
Make sure you Save As, then the print file will be saved as a different file separate to your original.