How big of a dance floor rug do we need?

My quick internet search estimated a guest list of 60 and about 20 dancers at a time would mean that you should get a 10'x10' dance space. So, with your list of 50, a 10'x10' would be plenty of room.

I think the height of the grass would determine if it's a tripping hazard, but it would definitely be safer and more stable if you placed the rug on a hard surface.

A rug isn't really safe for dancing. I would look into having a hard dance floor, otherwise you're gonna have some sprained ankles...

I honestly think a rug would be a hazard even if it were on a flat surface. You might want to consider renting a movable wood dance floor instead.

A rug is a bad idea for dancing, it can bunch up and people will trip.

I agree with the other PPs. A rug is very much a safety hazard as it easily bunches, plus it would be difficult to dance on.