If you wish to produce a very large print with the maximum amount of detail, so that each image in your collage is as clear and detailed as possible, you should do a couple of things. First, the direct, "native" print size for an image 5120x2880 pixels in size, printed on a Canon or HP ink jet (native 600ppi resolution) at the "standard" 300ppi is:

> 5120 pixels / 300ppi = 17.06"  
> 2880 pixels / 300ppi = 9.6"

Conversely, on an Epson printer, which use a unique 720ppi native print resolution, the "standard" ppi would be 360, resulting in:

> 5120 pixels / 360ppi = 14.22"  
> 2880 pixels / 360ppi = 8"

There are a couple things to note here. First, if you have a collage with many images, neither of those two print sizes are very large. Second, if you really want to preserve the detail in your image, you will want to print at a higher resolution. Preferably, you would print at 600ppi for Canon/HP, or 720ppi for Epson. At maximum resolution, your physical print size drops considerably:

> 5120 pixels / 600ppi = 8.53"  
> 2880 pixels / 600ppi = 4.8"

> 5120 pixels / 720ppi = 7.11"  
> 2880 pixels / 720ppi = 4""

If you really want to print something poster size, you will need more resolution. It should be noted that when you print on Canvas, the maximum amount of detail that can be maintained will generally be less than if you print on another form of paper. You mentioned that you are printing a collage, which I assume means you merged together multiple images. If the original images are large, you might want to rebuild your collage at a higher native size. For normal papers, I would print at 600/720ppi, and for canvas, I would print 400/480ppi. For a very large print, say 48"x27" (thats four feet wide), you would need an image that was at least this large for normal paper:

> 48" * 600ppi = 28800 pixels  
> 27" * 600ppi = 16200 pixels

> 48" * 720ppi = 34560 pixels  
> 27" * 720ppi = 19440 pixels

Or at least this large for canvas paper (note that, while these resolutions do not integrally divide into the native resolutions, they do divide "nicely" at 1.5x):

> 48" * 400ppi = 19200 pixels  
> 27" * 400ppi = 10800 pixels

> 48" * 480ppi = 23040 pixels  
> 27" * 480ppi = 12960 pixels

Generally speaking, for such a large print, I would recommend 300/360ppi. In the case of a collage, generated from other detailed images, however, I definitely recommend printing at a higher resolution. Every image in the collage can maintain quite a bit of detail, and while at first glance your viewers may look at the print from several feet and "take it all in", there is a lot more detail that can easily draw your viewers in for a closer look. Detail that is not normally present in a poser-size print of, say, a landscape. If you have a physical print size in mind, you can determine how many pixels the image will need to have to print out correctly at that size, without loss of detail. Simply multiply the size in inches by the PPI you intend to print at, to determine the pixel size of your image:

> W" * PPI = Wpixel  
> H" * PPI = Hpixel

Finally, it is important to know the native resolution of what you will be printing with. In the ink jet world, Epson is unique with their 720ppi native resolution, and 2880x1440 dpi print heads. Canon and HP, two other common printer brands, have a native resolution of 600ppi, with either 2400x1200 dpi print heads, or in the case of some Canon printers (such as their professional PIXMA Pro9500 II 13x19" printer) 4800x2400 dpi. Many dyesub (dye sublimation) printers have oddball native resolutions, which also usually match their dpi, which may be something like 320.19ppi/dpi. In the case of such printers, you will want to scale your image to exactly the correct PPI before printing.

Many professional print shops use commercial HP printers, while fewer use Canon or Epson. Regardless, you should try to find out what printers are used, choose a paper type, and scale or rebuild your image accordingly to get the best results. Make sure you manually scale your image to the right PPI and pixel size, to prevent the printer from scaling with unsightly artifacts.