I generally agree with Aart. A 12x18 image would be the standard image size without resizing. You could easily resample and resize the image to much larger size - 24x36 would be reasonable.
If you are resizing to more than a 50% increase in size, it generally helps to resize to an interim size and then resize again.
The challenge with resizing is that you can make any artifacts larger. Shooting at a low ISO helps reduce noise, but you need to watch your editing steps and use noise reduction. As with resizing, you would be better off applying multiple light sharpening steps with noise reduction along the way. It can also be helpful to apply selective editing.
I generally apply light noise reduction before resizing to a larger size then sharpen after resizing. If you are downsizing, you can usually skip the noise reduction step since artifacts are getting smaller.