I personally have a Humminbird 967c 3D unit which has a 8" screen. Have a 28ft Baha hardtop fisherman and have no problems seeing the screen from the back of the boat. Unit is mounted on dash up front. Have heard some really good things about the Lowrance HDS units. I don't think you need any larger than an 8" screen. Another thing to consider is the sonar types. Mine has the 433khz 3d sonar plus a standard 83khz sonar. 3d works great while trolling or simply moving forward to locate specific structure but is nearly useless while drifting as it has 6 narrow cone beams (16 degree) shooting left to right. While drifting you would only get a 16 degree bottom view, so I switch to my 83khz 60 degree cone angle while drifiting to get a much wider bottom view and fish finding. Hopefully not to confusing, just saying that don't get sold on high technology side imaging or 3D unless you understand it only works well if you are moving forward. Hope this helps.