Its going to be a 42,000 seat dome which is only 7,000 more seats then Sam Boyd Stadium but this is going to have a lot of stores and restaurants to it.
The Thomas and Mack center is 2nd only to Madison Square Garden in terms of revenue and they hope this new dome will pass MSG in revenues. This dome can convert into a basketball, boxing, soccer, rodeo, and concert arena. They want this to be a tourist attraction and try to lure a NBA or NHL team to Las Vegas.
Link from Las Vegas Sun