It was the paint pot which made me look twice - I thought it was a real hangar at first glance, until I saw the Tamiya paint pot in the second pic. great stuff Jim !
Concrete slabs could / can vary in size, but they tend to be in one of (or a mix of) a series of 'standard' sizes, based on the manageable dimensions of the timber used as shutters when pouring, and the capacity of the concrete mixer, and a rate of flow that again is manageable for the workers laying the slabs.
Approximate sizes (in feet) are 6 x 8, 8 x 10, 8 x 12, 10 x 12, and square slabs in multiples of two feet, for example, 6 x 6, 8 x 8, 10 x 10 and 12 x 12.
Obviously, where space dictates, or where there's an angle, such as a bend in a track, smaller slabs might be laid at the edges (or center), so an area might be made up of a combination of the common sizes, and combining two or three sizes on your base will give a more realistic appearance, and avoid a cen that looks too uniform and artificially 'tidy'.