Resolution is a primary factor in what kind of copy you can get from your original photo. There are a number of technical tutorials online about the specifics of resolution and what that means, but for the simple purpose of getting a copy of your picture, you don’t really need to know everything there is to know. I’m going to make it simple.
The ideal printing resolution for an image file is 300 ppi (pixels per inch). Take the size of your photograph and multiply each dimension by 300. This means that if you want a good 4×6 copy of your 4×6 snapshot, your image file needs to be 1200×1800, or 2.16 mb (megabytes).
If you’re scanning your own photo, take some time to look around in your scanner software. Most scanners have a default setting of 100 or 200 ppi; this is because you don’t need the resolution to be as high for posting your picture online, which is what people do with their scans most of the time. If you search the settings, however, there are probably options to set it to a higher resolution.
You may find that you can bump up your resolution to 400, 600, 1200 or even higher. These settings would be used if you want your copy to be larger than your original photo.
For instance, let’s say you want to make an 8×10 from your 4×6 snapshot. How much larger is that? Let’s do the math: 4×6=24; 8×10=80. 80/24=3.33. You’d want the resolution to be at least 3 times greater for an 8×10 than for a standard 4×6 copy. 300×3=900, so you’d probably set your software to use the 1200 resolution (unless yours offers the option of 900). What would be the minimum file size to get an 8×10? Multiply 2.16 (the original file size for a 4×6) x 3.33. Your file would need to be at least 7.19 mb to get a good 8×10 print from your original 4×6.
Now, “good” means different things to different people, and different printers have different minimum resolution thresholds. Some printers with software able to compensate for low-resolution files will yield a perfectly good print with a 200 ppi file. And many people won’t be able to tell the difference between a 100 dpi print and a 300 dpi print. You’ll have to use your own judgment about what’s acceptable to you. For best results, though, just remember: 300 is the magic number.