Spoilers: Daily Integrity Checks are not only a best practice, but they also provide a snapshot of global sizes and density.
Update 2024-04-16: As of IRIS 2024.1, Many of the below utilities now offer a mode to estimate the size with <2% error on average with orders of magnitude improvements in performance and IO requirements. I continue to urge regular Integrity Checks, however there are situations where more urgent answers are needed.
EstimatedSize^%GSIZE- Runs %GSIZE in estimation mode.##class(%Library.GlobalEdit).GetGlobalSize(directory, globalname, .allocated, .used. 2)-
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