as I have been told often - size is not important.....
Its the viewing distance that is what effects the size you can print to - generally the bigger the image, the further away you look at it.... go to a billboard and get as close as you can - you'll see it's almost like lego...
If it is just a photo, with no type you'll get as good quality supplying the file you have without resizing (you might have to crop to the correct shape) - if you are adding type resize to the fininished size, sharpen, then add the type.