For those of you with Photoshop, here's a simple tutorial for resizing images for print in Photoshop.
Much to my surprise, I discovered that Photoshop (CS2 is what I use) has a wizard for resizing images to print - something I had not seen before since I never ventured into the "Help" menu, which is where you'll find it.
I am currently printing my own images (up to 13" wide X 44" long) on an Epson R1900 and the prints I have resized using this wizard come out beautifully on that printer.
I have not yet sent these off to a print-house to test them, so before you decide to sell anything sized this way, you'll need to obtain a test-print from the printer of your choice.
Download the tutorial in PDF format from HERE
The pdf contains screen prints of the steps, so it should be simple for anyone to follow.
Since I use Photoshop CS2, I have no idea if this option is available in prior versions, or if it is available in Photoshop Elements. You'll need to look in your programs to see.
Starting with the highest quality of image you've got will help result in the best quality of print.