What boat is it going in?
What's the helm area set up like?
Where, how, do you plan on mounting it?
Why?
If a tiller steer, a 12 might be a bit big and 'hang out' into internal space a bit too far.
If a side console, do you have room to mount it? And will the screen be easily visible from fishing position vs driving position?
If a cabin / cuddy cabin type boat, can it be seen from where you will be fishing vs driving position viewing angle.
Toys are great. Shiny new electronic toys are awesome! But, sometimes practical reality needs to be considered first.