Answer: There are a few tricks to making small files bigger. However, it all starts with a good quality high resolution image. You can't take a 5" x 5" file that is 300ppi and make it 120" x 120" and still have a clean image. The resolution on that file would drop to 12.5ppi and quality would suffer greatly (nor can you save a low resolution image from a web site and make it huge!).
Therefore, you need to be mindful of how low your resolution drops as you scale an image. We have found that photographs can be printed at 72ppi and still be clean and attractive on large format presses. But that doesn't mean you can send in your files at 72ppi and everything will be perfect! Rather, you need to use a combination of programs to make everything work.
The common practice is to use a program like Illustrator or InDesign as your layout tool. Then, you place the lower quality images (pre-scaled) into the document. Those files will print cleanly (assuming your file is clear on the screen at 100% zoom) along with any higher resolution files used. Using Photoshop to scale your images properly is key.
As you select the "image size" tool to scale your image up to the print size desired, you need to uncheck the "resample image" check box. That will allow the resolution to drop naturally as your file increases in dimensions. Try not to let an image drop below 45ppi as this occurs.
Then, once the resize is complete, recheck the "resample image" check box and force that file back up to 72ppi. You should now have a relatively clean, larger image.
Using the noise filter and gaussien blur filter in small amounts can clean up any imperfections.
NOTE: these rules only apply to photographs. Any images that contain text or logos will not print cleanly using this method. The 100ppi rule is firm when dealing with resolution. Any raster images that contain logos or text can only be scaled up to the point their resolution drops to 100ppi. Beyond that, your text and logos will print pixilated.
View our handy Resolution PPI Calculator Formula PDF for details on how to prepare your file