How large can I make this photo?

955×690. Seriously, that’s it. Unless you want to blur it to hide the pixellation? But the quality will dramatically decrease at even a pixel bigger than the constraints you provided.

Technically speaking, any enlargement will compromise the image quality. That’s true for any bitmap-based format, but especially for those, like JPEG, that utilize lossy compression algorithms.

[Update:] A little looking around and I found an article that sounds kind of promising. Depending on the kind of image you’re working with, you could try converting it to a vector-based image.

Such a concept will not work well with photographs, but if your JPG is of some kind of line-art (like the example used in the article) then you might just be able to pull it off.

If you must enlarge it, first save it as a TIF or other lossless format. Then, try running it through Genuine Fractals software from onOne Software. It works miracles in enlargement, much better than PhotoShop.

Thanks @steel!

Just fyi: onOne Software’s Genuine Fractals 5 is $159… not a bad price if you do that kind of thing a lot. You’ll need the $199 edition if you want to resize CMYK, though.

The site says of builtin enlarging algorithms used by Photoshop:

Bicubic, along with its newer variants, Bicubic Smoother and Bicubic Sharper, work by averaging a small group of neighboring pixels together to determine the color value of the new pixels to be added. While this method is fast, it does not distinguish edges so there is a uniform loss of sharpness and detail across the entire image.

And then, of their technique, they say:

The patented, fractal-based scaling algorithm found only in Genuine Fractals is superior to any resizing algorithm found within Photoshop (bicubic variations) or in any other third-party plug-in or stand-alone application. There is some serious heavy-hitting math going on behind the scenes but basically Genuine Fractals examines your image for naturally repeating patterns and once it finds them, uses them to increase your image to any size you desire. The result is an enlarged file that maintains the edges and the color values from your original file without introducing any artifacts, blurring or halos around edges.

I’ve got Genuine Fractals, as I do massage a lot of pixels. It is pricey, but if it saves me an hour it is worth it. I’ve blown up 8×10s to poster size with miraculous results.

Yep, Genuine Fractals impresses me too. Even with rather small images it often guessextrapolates (my word) well.