Art 2/3 the width of that side of the sectional. Look at art.com minted.com and other sites to see what you like. It should be hung 5-7" above the back of the sofa. Lower the trio on the other wall.

As option you can use some console table behind with couple lamps on it.

Thanks! What do you think about shelves to add some art/other pieces? I'm thinking a plant or two on the one by the window.

Some option

I don't like shelves Usually end up messy looking

I suggest a big arc floor lamp at the corner of the sectional, and take the existing prints you’ve got and put them in very large black frames, with matte, no glass. Place a tree in the corner next to the window. I think the kiddie table and chairs are adorable and if the kids will let you, put them in front of the sofa for a coffee tabl. Put a storage end table where the kiddie table is now.

George Kovacs Portables 1 Light Arc Floor Lamp in Chrome with White Marble Base · More Info

Thanks everyone. Any suggestions for where to buy frames? Looking for something maybe just a little more reasonable than west elm, etc where they're up to $200 for a 24x36. Also don't want to buy something super cheap/low quality.

Look at IKEA frames

or simple shelf with wall arts