What's your goal?....

More boats at the Worlds? Or do you want to change the nature of the competition a bit... eg... two 50 boat fleets.

Your question is also location dependent... How many locations could scale up for more turnout.

Finally, How much demand is the there for slots over 100 boats. Seems to me that the most recent US and Aus worlds just made it to 100 boats and the intercontinental sailors could race if they wanted. Your question probably pertains to the EU Worlds where the demand for slots is high.

The Tornado Worlds split into Gold and Silver Fleets once when sailing on a smaller lake and they just could not manage the race course. Generally positive feedback.