Sizes were estimated to be:

The Forest Moon, Endor ~ 5200 km (3120 miles) radius
The Death Star ~ 450 km (279 miles) radius
The Executor Class Battle Cruiser ~ 17 km (10 miles) length
The Star Destroyer Battle Cruiser ~ 1.7 km (1 mile) length

The sizes of the Imperial Starships are beyond my imagination. A couple years ago I designed an interstellar cruiser, Starship Generations, with a spherical Habitat of radius 250 meters. It was composed of 50 concentric cylinders, nested one-inside-another, and of varying lengths, all enclosed inside a spherical shell. The result was a habitat with over 1300 hectares of surface area for growing food and housing 10,000 people. There was a 50% excess margin for agricultural area, and 92 hectares would have housed each crew member comfortably within a 1000 sq. ft dwelling.

How many people would a 900 km-diameter Death Star accommodate? Perhaps the entire Empire! And the size estimate might explain why the Millennium Falcon was able to fly so fast inside it, and the huge chamber at the center that housed the Reactor (that was a cool scene by the way when it was destroyed).

An interesting plot might have been the Empire's intention to strip mine the Moon to complete the construction of the Death Star. There wouldn't have been much left of the Moon.