The size of your venue really depends on what you think makes the most sense for your location and what kind of celebration you envision. Consider attendance at gatherings similar to World IA Day in your location (i.e. meetups, professional gatherings) and the amount of attendees you are comfortable managing. It is also worth considering what venues are more readily available.
Renting a venue is often the one of the highest expense reported for a World IA Day celebration. Look into free venues, such as libraries or other community spaces, to eliminate this expense.
TIP: In some cases, companies who are targets for sponsorship may be able to donate space instead of making a financial contribution. Reach out to these companies to see if they would donate their space in exchange for sponsorship perks.