I like larger pieces over the sofa. You'll be viewing them from across the room since those on the sofa will have their back to it. There are a lot of good examples on Houzz of gallery walls with smaller pieces so guess that works too.

I think you need to add a lot of color. It's quite neutral now.

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Here's some discussions about gallery walls

https://www.houzz.com/magazine/a-gallery-wall-for-every-personality-stsetivw-vs~538039

https://www.houzz.com/discussions/5893039/can-i-get-some-help-with-a-gallery-wall#n=7

https://www.houzz.com/discussions/4277141/wall-gallery-from-just-above-floor-up-or-starting-from-above-couch#n=7

I agree with katinparadise -swap the picture with the mirror. Create a gallery wall with the mirror above fireplace mantle.

Some idea

or

Thank you so much! This is exactly the look I want, but had no idea how to put together. I love the colors you selected. Thank you for taking the time and showing pictures of what my new gallery wall will look.

The thing about mirrors is they can reflect things you don't necessarily want to highlight. So, I'd be careful about where you do place your mirror.

I don't see an appropriate space for that mirror in your photos; its just too small. The large piece of art is too wide and anything hung there should be no wider than the firebox opening below. Any piece of furniture under that art should be at least 25% wider than the art.

Thank you for your response and for the great information.