I haven't seen any formula for estimating bitmap index size. I have seen a formula for regular B-tree indexes, but that formula is very complicated, especially for an estimate. To be honest, most DBAs estimate sizes by loading a small, representative set of data, then build the bitmap index, and check the size. For instance, you might load 1,000 rows of data and create your bitmap index. Then check the size of your bitmap index. Now extrapolate for your expected data volume.
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