How big can I make a good quality poser of a photo?

I believe that it depends on the DPI (dots per inch) of the printer (you will need to find this out). If, for example, your printer was 300 × 300 DPI, then for every inch (every 2.54 cm) it can print 300 pixels.

So, the picture is 600 × 300 pixels as you say (which is actually quite small), and you were to print it on a 300 × 300 DPI printer, to have the most accurate and best quality printout at 1:1 ratio, the physical image will only be (600 ÷ 300) × (300 ÷ 300) = 2 × 1 inches (approx. 5 × 2.5 cm). The higher the DPI of the printer the larger the image can be (for example a 600 × 600 DPI printer could print that same image double that size and it would look just as good).

This means if you want to make it poster size you will have to use a program like Photoshop to scale it much more larger – you will probably need to experiment a bit because there is no way of finding out how large an image can be scaled before it is of unacceptable quality except by judging it yourself.

If you are using the 300 × 300 DPI printer and want to print an A3 sized (297 × 420 mm, or approx. 11.693 × 16.537 inches) version of the 600 × 300 pixel photo, [this is where it gets hard because printers are measured in inches whereas standard paper sizes are measured in millimetres] you will need to resize the image to about [we can’t get it exact because of the difference in measuring systems, but it’s close enough] 3508 × 4961 pixels, whether or not this looks good is for you to judge, but I would guess it would look very blurry.

Sorry if this sounds complicated (all those numbers!), if you still need clarification feel free to contact me.

In general, an image printed at 300dpi will be “perfect”, typically 200dpi will be indistinguishable (just a little less crisp), and I have printed large scale posters at around 80dpi with no real problems—because the poster is so large, you have to stand back a ltitle bit to see the whole thing.

That said, if the image was taken with a 7MP camera at full resolution, the image should be around 3072×2304. If the image has been scaled down, or was not taken at full resolution, you may not have this many pixels.

3072×2304 images can be printed to 8×10 at 300dpi, 12×18 near 200dpi, and 20×30 around 100dpi.

So… double check your camera settings—you may be not shooting as high resolution as you think you are if you are getting 600×300 pixel images. (0.18 MegaPixels vs. 7MP)

Magazines typically print their images at 300 dpi. So if you are looking for a magazine-quality poster, you aren’t going to get it—the largest image you could manage would be a 1“x2”. When you enlarge a digital image, you are basically adding pixels. Doubling, tripling, quadrupling the existing pixels so that what you end up with is a very blurry, very pixelated image.

If you saved the image straight from the camera, you might have changed the resolution accidentally. Look back at the original if you have it, and check your camera settings as bpeoples suggested.

You can use a program like Genuine Fractals from OnOne Software which will do a better job of adding pixels to print larger version of your image.
The thing you should do when printing larger is to resample the image to match the output dimensions and at the same time keep the resolution to that of your printer (240dpi or 300dpi depending on vendor). Use photshop or GF for this.

that’s way too small. Like above, check the settings on your camera, it seems that you are not using the 7mp at its highest resolution. Forget programs & math…you won’t be able to get a poster out of that size. Maybe a decent quality 8×10 at best.

It all of course depends from which distance you are going to view that image. If you do resample that image from that small size to something bigger you will be able to get an alright looking, not great looking, photo that will look alright from a distance.