The main telescope itself is quite impressive in scale, as you can see compared to the spiral staircase in this photograph. Don't leave your jacket in the car if it is a cold morning, because the dome is kept unheated. This is both because it would be too expensive to heat the dome, and to improve the astronomical viewing conditions by avoiding the atmospheric turbulence that would be caused by warm air escaping the open dome by convection.
The architectural details are fantastic and full of astronomical symbolism, some of which will be described by the tour guide. It is pretty neat how the lobby hasn't changed very much from how it was at the end of the 19th century.
If you take a tour, make sure to keep an eye out for Spiderman hanging out in the rafters of the dome!