I did a web search on "room capacity" and this was the first hit. I don't really know how reliable it is. However, if you do a search and find several that say about the same thing, I bet you would have a reasonable answer.

Here is a link that might be useful: Room Capacity Calculator

I was just thinking about this a moment now. I put into that calculator that you had 3,000 square feet of space (50 x 60) and it said you could get 333 people in there. Yup, you could get 333 people in there. They would be packed in like sardines though! 10 square feet per person is much tighter than I would be comfortable with. Some people and some dancing styles would accomodate that, but I personally would get a bit too claustrophobic for that. I think I would probably try to keep it to at least 30 square feet per person after you take out tables and such.

As an aside, I assume this is a hay barn. You might want to have some drinks and stuff set up outside of the barn. When your friends want to step out for a few minutes for a breath of air, it would be nice to have some seating and drinks out there as well.