I would agree 8" is about as small as I would want but you the 7 might just do if it is mounted very close to the helm.
Our chart plotter size was limited to the size of plotter we can fit into the helm cluster, I really wanted it built in rather than on a bracket due to the helm shape.
At 8" in front of you it is very usable but if it were positioned on the Navigators side it would not be easy to read particularly in split screen mode.
As has been said get the largest screen you can afford.