Prop Size ?

I am running a 10 x 13 stainless prop on a tohatsu 25 4-stroke and it runs fine with me and the wife but with anymore of a load it doesn't plane out well. What prop size should I go with for more power to lift the loaded boat better?I'm not worried about speed. Also I put a c m c tilt trim on it and I have more speed but I have to tweak the tilt trim a little more to get it right on top of the water.

Have a tach? That would be really helpful.
I assume it's lugging along looking for some rpms, correct? It's not cavitating and setting down?

I don't know how you would select a prop without knowing what RPM it's turning under load.

Bob thanks your advise.