Our guide to light sizing recommends subtracting 12 from the diameter of your table to find an ideal maximum size for a light. That would be 24" in your case, but the fact that you're worried about obstructing the view makes me hesitant to recommend the 25" unless it's a very open design, like just clear glass. The 18" is smaller but may be a better option because of your view.

U should look up online the correct size fixture for the room first that should help with size to pick. It’s something like u measure the width of room and multiple it by 2 or something and that tells u the perfect size!