The quality of being commensurate; the state of being able to be compared or measured by a common standard.
From commensurable (adjective) plus the suffix -ness, which creates abstract nouns from adjectives representing the quality or state of being that adjective.
This noun is so awkward and old-fashioned that modern writers would just use 'commensurability' instead—showing how English language evolves to drop redundant word formations.
Complete word intelligence in one call. Free tier — 50 lookups/day.