Do you have an interview there already? If you do, they should have sent you an itinerary for the day. You need to be at the school by 9 am, and the interviews and everything else end by 2:30.
It's very relaxed. You start by hearing from the dean, then some other facts about UNLV. They'll show you how many people have been invited to interview, how many have actually interviewed, how many have been offered acceptances, and how many have sent in deposits.
You'll go on a tour. Their facilities are awesome. You also have a financial aid presentation, which isn't as awesome, because UNLV is pretty expensive (due to NV budget cuts).
After lunch, you have interviews: one with a student, and another with a faculty member. My interview with a student ended up with us talking about college football and playing ping pong. Very relaxed and a good chance to talk with students and see what they think of the school.
Finally, your interview with the faculty. They try to put you at ease and just ask you a few questions about yourself and your application. It's very relaxed as well.
As far as class size, they have 80 spots. I have 'heard' that there are about 40 people that have sent in deposits, but I know of two that deposited at UNLV that won't end up attending there. In my opinion, you should have a decent chance to get in if you've been invited for an interview.