Just keep in mind that you cannot hurt anything by going big..... No overkill in this department (except may looks).
I had two 2"X4" on my 25' Sled and a 1" X 3" on the bottom of my nozzle..... Never had even the slightest pimple or blemish from the salt, but did go through 2 sets of the zinc electrodes.....
Grounding to the hull is a necessary evil, and as long as you have the zinc in place there is no problem. It is the salt and aluminum that together make the electolysis occur. Since Zinc is a an end conductor, if in place it will isolate the electolysis and take the damage (by giving up electrons), thereby saving your hull...... Keep the one on your lower unit in good shape too though...

Good luck!

MC