Library Eyes
you know, if you break it into smaller segments you can be flexible in your ways to mount on the wall.

SKL
In terms of color, maybe do some photos B&W to tie the collage into the room, but put the most important images in color so they pop. Maybe alter the colors digitally so the reds come out stronger. I would keep it big so you have more leeway in how you arrange the individual photos- 12" isn't much in terms of photos- you can only get 2-3 in that space depending on how you orient them and that will lead to a less-interesting layout. If it's bigger, you can have photos bridging over in more places. Do a dry run with the photos before you glue anything down and see how it's going. Maybe 2 24"x36" posters would work, but I think a 12" height or width would limit your arranging options.