When I'm restoring old photos for my customers, I'm frequently asked "Can it be enlarged?"
The answer is always "Yes". All that's required is for me to scan it at sufficiently high resolution. As long as I capture enough pixels, any photo can be enlarged to the side of a truck, if you wish.
(I will write about the technical aspects of "how many pixels is enough" in a future post.)
It needs to be clearly understood that nothing changes during enlargement. The quality of the photo won't become worse, nor magically better. It will be exactly the same photo, only bigger.
If it was a clear, well-focused photograph, it will be a clear, well-focused photograph at any size. Conversely, if it was a bit blurry and out-of-focus, it will be equally blurry and out-of-focus at any size. Not worse, not better, just larger.
Let's face it, not all of the old cameras had the amazing lenses and focusing capabilities of our modern devices (although some certainly did), so this is certainly something we need to consider. I guess it's fairly simple to foretell how a photo will look when enlarged - just hold it at normal viewing distance, then bring it closer to your nose. (I'm full of high-tech advice! Ha ha!)
If you wish to enlarge your old photo, go right ahead. Folk don't bring just any old photos to me - they bring photos that are rich with character and historical significance. And that trumps technical imperfections every time.