DVD is 720x480, but it can be widescreen. Each step in the workflow must be correct (source, Premiere sequence setting, export setting, and Encore settings). You should examine each step and make sure it is correct.
As one test, when you import your file to Encore, select the file you imported (not the timeline if you "import as timeline"), look in the Project Panel: what does it say in the DVD Transcode Status Column? (Should be "do not transcode.")
If you right click and pick Interpret Footage, what does it say for the Pixel Aspect Ratio?