Keep in mind the height of your mount. A 9" unit 2' from your face is going to look much larger and clearer than a 10" or 12" at your ankles.

I currently have a Helix 10 and 106SV on a Cornfield Crappie Gear double mount with 10" arms. I started with 6" arms and recently moved to the 10" length.

My next boat will likely have a 12-15" unit on the Beatdown Outdoors mount.