You could use either colored or black and white photos in your gallery wall. I'm more partial to color because most of the photos of me under the age of 5 are in black and white. You can fill the entire wall or just a portion--there are no rules! Here are some ideas:

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I think wall ledges for photos is far more interesting than simply hanging pics on the walls.

Technically, I would all this a "great room." It is the room that immediately follows our foyer. The fireplace is the focal point of the room, and it is used as a sit-down-and-chat area and a place to curl up with a book.

You have a high and long wall. I would use nothing smaller than an 8x10 ( just a few), or 11 x14, 16x 20 with a mix up to even 20 x24 center. Cut paper first in multiples of those sizes, arrange on the wall and decide how many of which frame size. Your matts should be generous, and I'd keep all the photos black and white. ......and all frames the same at least in wood tone, or black.

You'll have the impact, without the clutter of too many, or too many smalls on the large space. Experiment until you hit the arrangement that pleases you, keeping inches between relatively consistent for most of the arrangement. These walls can very easily look cluttered without some thought to size and arrangement.....

I like family photos in hallways or family rooms but if you must I like all one color and style of frame and as far as layout it will depend on how many wahat sizes etc. I too like ledges where the pics can be changed or new ones added without a big production