A common recommendation for printing is 300 ppi, so approximately 17x10 in. (16x9 is probably easier to find)

For a poster or canvas you can often go a little lower, into the 200-250 ppi range without too much difference in quality.

If you dropped all the way down to 160 ppi, then you could do a 32 x 18 in. print.

The best path, however, is to ask your printer what they recommend.

In regard to printing on poster vs canvas:

In this situation, it being a collage at an aspect ratio that you want to maintain, I feel that you will have better luck with a poster.  A poster will not have the texture that canvas has, but in this situation I think that would be fine.  Posters are easier to have [printed at a custom size][1], and you often have semi-gloss or high-gloss options.


  [1]: http://www.largeformatposters.com/Semi-Gloss-Posters.html