Ultimately the only one who can judge this is yourself. I'd suggest taking a great picture and going to
www.ofoto.com or a local printing service, then getting various sizes printed and what your eyes tell you is the answer. ;-)
My hunch is that you'll see everything up to 11" x 13" looking great, but above that at 16" x 20" you may start to see some pixellation. There are some resolution bump resizing tools you can try (I bought one, but I'm drawing a blank right now at what it's called) but ultimately there are limits to how far you can do with it.