The end of a shower rod often (it does not have to) lines up nearly with the inside of the tub opening, so that would be more than sufficient. Some people like it to be further out, but there are two ways to accommodate that: have a decent overlap, or use a curved shower rod. For aesthetic reasons, it is common to make that wide enough so your trim piece can continue to the floor whole, so the width would depend on the tile and trim you select (assuming you're going to use a trim, maybe a bullnose, there). FWIW, if the tub you want is available with tiling flanges, you're much better off using that. I bring this up because you mentioned shower. Unless you are going to use a circular shower curtain rod, you need to waterproof the tub edges, and while possible, it is MUCH better to buy the tub with an integrated tiling flange - your chances of a leak there are much diminished.
