I'm not a travel agent and would recommend that you contact Susan W directly, or another agent of your choice, who has knowledge of Seabourn ships.
There are many questions that they will need to answer to help you make a choice. Have your parents sailed before? If so, what ship/experience did they like the most and why. Do they prefer a small ship or a larger ship? Ask what the difference in experience would be if you have not sailed on different sizes of ship. Would they be content with a more formal dress code for dinner or would they prefer more casual dining? Do they want a variety of dining options? The itinerary might be important as smaller ships often dock in areas where larger ships cannot dock. Any cruise agent that you contact should help you wade through the massive amount of information available as well as provide you with information regarding cabin locations on a ship, what is and what is not included, etc.
What a fabulous anniversary present a cruise would be and I hope you find the right cruise!