In my experience, big crowds are pretty easy to play for. The largest I've played for was maybe 1,000.I've heard several famous musicians say that playing for a huge crowd is actually less intimidating because you don't even notice individual people. It's just a massive sea of humanity. YMMV
Anything over a few hundred becomes one crowd. Playing for a crowd is easy. It's the gigs where three people show up that can be tough, because you can see them watching you. It's like the less people I play for is more difficult. Playing love songs for my girlfriend is when things get the hardest.