This should be one of the first things you verify when sending/receiving artwork. You want to check two important details within the art, size and resolution. To do this, in your menu up to, go to Image > Image Size... image size in photoshop Ideally you want a resolution of 300 pixels per inch, and then the width and height would be true to print. Which means you should aim for 300 PPI. Note that you can use the dropdown and select your unit of measurement. If it's on pixels, you can change it to inches, as above.
Uncheck resample. This will keep the true number of pixels inside the amount of inches you choose. If I change this to 10 inches, then you will see my resolution becomes 300. So, many more pixels are are within each inch, thus providing more detail per inch and keep our edges sharp for print. In this case, the math works out. But you must note, because Photoshop is a raster based program, you can not just scale images up. This will severely affect the quality of your art. At 300 PPI like this, I could bump up from 10 inches wide, to 12 inches. There is a little bit of wiggle room. But if my image was only 150 PPI, just going up 2 more inches will start to show and degrade the image. You can however, shrink downwards without degrading. For example if you had a large back print with a matching left chest print. The most important part of this to take away, and that applies in the real world, is that you can not take random images from the web, open them in photoshop, and then bump them up to 12 inches big and 300 DPI and expect a good print. This you could only do with vector artwork. As we recommended 300 PPI above, 200 may be passable. Anything less and you are flirting with a possible bad print.