This is a screen shot of File/New dialog box in Photoshop CS. This is exactly the dimensions of a standard aspect DVD menu. There is a preset that will set you up just right. Important note - the pixel aspect ratio selection at the bottom... TVs (and DVDs) use rectangular pixels. Computers use square pixels. The Photoshop CS will work with both. If you're using a pre-existing image - with square pixels (the default) and you crop it off to 720x480 pixels, you will get some vertical distortion when you import it into your DVD software because the square pixels are being treated like rectangular pixels.

Before they made me they broke the mold!
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