The native resolution of a NTSC TV is something like 648x486 interlaced. That means that you have 30 frames a second, and each frame is composed of two half frames (called fields), one of which contains the even numbered scanlines, and one contains the odd numbered scanlines. The closest Dreamcast video mode is 640x480, and that's what you should be using if you're going to output to an NTSC TV.
An NTSC TV can not display anything above that resolution. The height of the image is fixed, and can not be changed. You can decrease the height of the image, but you need to add blank scanlines to do that. You can alse decrease the width of the image, but all that really does is stretches the pixels, and you can not get a greater resolution that way.
The native resolution of a PAL TV is higher - 720x576 interlaced. As before, the height (number of scanlines) is fixed, and the width (number of pixels) can vary as long as it's below 720.
If you need that kind of resolution, you'll probably need to re-think what you're doing. There is no way any TV will be able to display that level of detail. It is simply not possible. In addition to that, a TV image will always appear slightly fuzzy because of the interlacing effect. Only half the image is being transmitted at a time, which means that things smaller than one pixel with either flicker, or be blurred by the Dreamcast's graphics hardware.
Try shrinking the graphics down a little.