Steam Game Covers

The reason it's not good to resize the template is because it'll affect the print quality, especially if you're using a laser printer. Also resizing the image will throw off the print scale. Your covers may print out too big or too small to fit into a CD/DVD case.

What's the minimum res for covers and discs, for future reference?

Just don't edit the template in any way at all. You'll have to resize your source images and if their resolution is too small, they'll look blurry on the cover. I don't think there's any specific minimal resolution, just make sure you use the best ones available and that they're not too small.