Only thing I would worry about with a bigger tank is getting a bigger compressor. Even with a 100% duty cycle "good" Viair compressor, it takes 10-15 minutes to fill the tank from empty. Going from 150psi back to 200psi takes 3-5 minutes. If you have too small of a compressor, you're going to work it too hard.

Regarding the solenoid, you won't have to worry about it. Think of it as an actuator--its only going to have power to it for a brief period of time (however long you're pushing the honk button). However, the larger diameter solenoid you go with, the more air you're going to flow and the louder your horns are going to be. The two things that will make the horns louder -- psi and air flow.