Union Square is big, but it's pretty straightforward. There's no confusion to be had. It's just a park and vendors are lined up along one part of it.
The bigger issue is how crowded it is, but then, I avoid that by going on Mondays and Wednesdays and even Fridays. Saturdays are rough. Saturdays are perfect for going to the calm, cool Sunnyside greenmarket where you don't have to get there super-early to get things like sour cherries when they're in season, or squash blossoms.