I don't think that 12 x 16 is very large for 35mm, especially if you are talking about paper size and not image size. It might be a little large for 800 speed film, if you don't want grain to be a part of the look and the feel of the image, but 400 speed film can be fine for that size, especially a pro film like Portra 400. If it's really a problem for you, I think you would need to use a larger format to be satisfied, not a slower film. Using a tripod and overexposing a bit never hurts.

-Tim