When it comes to printing large, the native image size out of the camera doesn't mean much. A 60x20 inch print is very large, and print resolution is measured in PPI, or pixels per inch. Even the highest resolution cameras of today, such as 18mp-24mp sensors, do not produce enough native resolution to be printed that large...most top out around 17x20 native. Your physical print resolution for a 60x20" print is 14400x4800 pixels @ 240ppi. For reference, thats a 69mp image...about three times higher resolution than the highest resolution DSLR sensors on the market today.
Your going to need to do some digital enlargement to achieve that, and you'll need to do some careful sharpening or use specialized tools to do so without softening the image too much. You will also want to make sure you know the type of printer that will be printing your photo. It is important to keep the image resolution relative to the native print resolution of the hardware you'll be printing with...otherwise the printer driver or rasterization software will do additional scaling for you. That could (and often does) compromise the hard work you put into careful enlargement in the first place.
Epson ink jets, including the commercial variety, have a native print resolution of 720ppi (note, print resolution and print DPI, or dots per inch, are not the same thing...DPI in print is usually quite a bit higher, such as 2880x1440.) The standard resolution for most other ink jet printers, both both personal and commercial, tends to be 600ppi. You will want to make sure that you resize your images to an evenly divisible ppi of the printers native, so that no additional resizing is performed during a print. For Epson printers, you will want to use something that divides 720 evenly, in which case 360, 240, 180. For other ink jets (so long as they are not unique devices with unique resolutions), you want something that divides 600 evenly, in which case 300, 200, 150. The difference between the two is minor, however you may wish to choose 360/300 depending on how you intend to mount the print, and how close your viewers will be able to get.
When it comes to actually choosing a print resolution, it will really boil down to how much original resolution you have, how much larger the print will be, how close you want your viewers to be able to view the print from, and how much effort you are willing to put into scaling and sharpening to preserve the necessary detail level. If you are going to hang the print in a location that precludes closer viewing, go with the lowest resolution...180/150ppi. That would require enlargement to 10800x3600/9000x3000 pixels. If you intend to hang the print in a location that allows your viewers to get within 4-5 feet (say middle of a standard wall behind a couch), you'll probably want to use 240/200ppi, which requires an enlargement to 14400x4800/12000x4000 pixels. If you intend to hang the print in a location that allows your viewers to get within 3 feet, and want to keep drawing them in with more detail, you'll want to use 360/300ppi. This latter scenario is more rare, and is only really necessary when you really want to draw your viewers in. In this case, you need an enlargement to 21600x7200/18000x6000 pixels. I have two walls in my house along hallways that do not even allow the viewer to get more than a few feet from the print, and I opt for 300ppi or more for any prints that hang along these walls.
It should be noted that if you do not have much resolution to start with, say 8mp or less, then there is no point in trying to preserve detail with 300ppi. With such a low starting resolution, your some eight times lower than you need for such a large print. No matter how you massage your pixels, they will never enlarge well enough for a 300ppi print...or even a 200ppi print. Your better off sticking with 150ppi, and saving yourself the trouble. If you are not starting out with 16mp or more, maybe even 18mp, you might not want to bother with 300ppi either, and just go with 200ppi. Once you move beyond a 3x enlargement, its extremely difficult to preserve fine detail. You might be able to handle a 4x enlargement from 16mp to 69mp, but its going to require a lot of effort. Finally, don't bother enlarging beyond 150-200ppi if there is not much fine detail to start with, or if fine detail does not matter. Landscapes, macro shots, bird and wildlife and the like usually have a ton of fine detail. Architecture, sports, most man-made objects, portraits, etc. generally do not contain the kind of fine detail that you need or want to preserve, so stick with the lowest resolution and save yourself time and effort.
I have written a small study on using iterative bicubic scaling to achieve extreme enlargements here on PhotoSE: Emprical Study: Extreme digital upscaling. You can use iterative bicubic to preserve fine detail, or you can try using one of the several enlargement programs tested to preserve fine edge detail (S-spline and fractal scaling do well maintaining edge definition, where Bicubic breaks down.)