You need to reduce the size (in pixels) of the digital photos. For example, if you took the photos at max resolution with a 4 MegaPixel camera, you absolutely don't need to put them in iDVD at this high resolution. When displayed on your TV they aren't displayed at this resolution anyway, since your TV can't handle it. I'd say resize each photo using a batch resize function (in Graphic Converter for example) and reduce them by half to 2 MegaPixels in size. I'm pretty sure even 2 MP is overkill for a TV, but it's a start. Your target here is to get below 4.1 GB in size for all the media you are putting into the slideshow. You also need to figure in some overhead for creating the slideshow, so I'd shoot more realistically for 3.8 GB or less for your total media (especially if you have some animated themes).
If anyone knows the exact resolution a NORMAL TV can deliver (in pixels), I'd like to know that myself. That would be the maximum pixel size you would need to have per digital photo without losing any quality.
Also, did they ever up the max number of photos you may have in a slideshow from 99? That is a pain. It really needs to be higher. Sometimes creating a second slide show to get around the 99 limit isn't something you want to do.