A digital print is made up of millions of dots of 4 colours. (Cyan, Magenta, Yellow and Black). 300 dots per inch (300dpi) to be exact. Any images that are used in your artwork must be a minimum of 300dpi to achieve a crisp quality print.
Your computer screen's resolution is 72dpi so an image that you see on your computer screen would print a lot smaller if it was changed to 300dpi and printed. Be wary of sourcing images from the internet as these are all optimized to be very low quality images at 72dpi.
Text and some special images (.ai, .eps) come in a form called 'vector' These objects are made up of a series of points and curves. Vector objects can be scaled to any size because of this and they will still print at a high quality.
It's hard to change the resolution of an image without a professional graphic manipulation program like Photoshop, but there is an easy way to find out how big an image will print at 300dpi.
Step 1: Find out the pixel dimensions of your image (windows will tell you this in the image properties)
Step 2: Divide the pixels by 300 and this will give you your print size in inches.
Step 3: Multiply your inches by 25.4 and this will give you your print size in millimetres.