One popular metric is the event’s "carbon footprint."
The graph (representing a large citywide event with a substantial expo) shows that the largest contributor to an event’s carbon footprint is the fuel used by attendees to travel to the destination (fuel represents around 80%). And don’t forget there's freight being shipped along with them, representing another 5-15% of an event’s total carbon emissions.
This means you don’t have to understand carbon footprinting and the science of greenhouse gases and climate change to make a huge difference, when it comes to sustainability; nor go digital; nor figure out if your printer uses vegetable-based inks.
It simply means you need to select an event location that is close to attendees and, ideally, accessible by car or train.
You’re likely already doing that, so keep it up and feel good that you’re not only increasing your odds of greater attendance, but having a positive effect on climate change from the comfort of your office.