You can scan the photos at 300 to 400 dpi and it will be way more than you need. It can still be good to scan them at 300 to 400 dpi just in case you ever want to print them out.
iMovie will reduce them to 1920x1080, or about 2 megapixels.
If you make a DVD, they will be reduced to 640x480 or about 0.3 megapixels.
You really don't want to watch a DVD on a large screen TV. I would suggest using an Apple TV, or some TVs allow you to insert an SD card with full resolution photos.