I don’t do that a lot. I have played in a Blue Skies game pitting 1/300 airplanes against 1/1200 ships on about a 40mm hex grid. That worked well.
How many model airplanes do you want to attack a ship in the same turn? Are you playing on a hex grid? How many model airplanes are allowed to attack in one hex in one turn? Or how many model airplanes can fit into the length of the ship model? Once you know these factors, it’s just arithmetic to determine how big a ship model you need to use.
You'll shoot your eye out, kid!