I think they should be similar (as should the colour of the frames), but you could also put two smaller pieces on one side to balance both sides of the shelf. That said, the shelf is somewhat small for that particular wall. Are you adding upholstered chairs, etc

Oh, and meant to add -- using newspaper and painter's tape, you can cut out and hang various sized "frames" to see what works.

Your bookcase is too small and the wire is distracting. Can you run that piece horizontally instead of vertically? I don't think I'd hang anything up until the space is better defined. I would also add that spring will bring on a lot of art festivals and you might find something you really like that will be original and that will support your local artists.

The reason people are saying the bookcase is too small is because it looks as though the bookcase is meant to be the centerpiece of the wall. When you think of a centerpiece, you think of something beautiful that catches your eye and commands a presence of some kind. Do you get that feeling when you look at this bookcase? If not, search for something else to use as your centerpiece on the wall. You can still use a bookcase, but use one that goes all the way to the ceiling and is very wide. Or, you can install shelves on the center of the wall that span the entire height of the wall so that they accomplish the task of commanding presence. You can also get three tall bookshelves and stand them up side by side to give the room a rich look.

I hope this clarifies peoples' comments and also gives you some ideas for solutions. You have some nice elements in the room to work with.