If it's the four stroke, and the Seagull Harrier 3D, I suggest the 14x6 or even a 15x4. The "3D" planes are usually not intended to be fast so that typically points you to large diameter and low pitch.

Prop selection seems to be voodoo sometimes. And it can be personal preference. I liked the way a certain prop worked on my trainer. Another pilot tried one on his plane and hated it. Similar plane and engine, just different likes/dislikes.

If it's a different engine or plane, ?????????????