Alternative dialogue words you could use instead of the word…

Plain

As a noun

Specifically to mean wide open space or area

Amplitude

Breadth

Compass

Depth

Distance

Domain

Emptiness

Expanse

Expansion

Extent

Extension

Field

Immensity

Latitude

Length

Magnitude

Range

Reach

Scope

Sheet

Span

Spectrum

Sphere

Spread

Sweep

Territory

Void

Waste