My thought is that the bigger the TV, the better for the immersive TV-watching experience.

I'd hate a TV over a fireplace, but you probably have your (good) reasons why you want to do that.

Show a picture of the room. Design is visual. I would opt for not having the TV over the FP.

We need pics of the space please .

Here is a 55" TV over a 60" mantel, and a 70" wall.

Best way to see it in your space is by taping it up and looking at it.

This is the 48 inch model

The 48" is totally fine. I wouldn't go any larger than 50".

Thank you for your feedback!!

Do you think it would look better mounted above the fireplace or recessed to the right on custom built in bookshelves?

You don't have much choice do you?

It would probably be a good idea to have the bracket that drops down when in use.

Scale to firebox, not mantel. The one you're holding looks fine, as long as you don't mind looking up all the time.

Thank you for all your advise and suggestions! Another option would be to have a custom bookshelf to the right of the firplace and place the TV there (see pics).