blockSIgnature events are just listings of eventIDs. Therefore a block signature event is just a bunch of numbers like 45:35 with spaces in between. Therefore the size of the block signature database has no correspondence with the size of the original events, making this difficult to estimate in bytes.
However, each event requires a price of around 8 bytes in the text, precompressed, and each event requires a hash and indexing-related overhead. I would guesstimate that each block, in in the current rawdata format and tsidx format, is going to cost on the order of a kilobyte. Empirical data is obviously vastly superior to my guesses.
There's therefore the obvious tunable here which is the blockSignSize. I think anyone using this feature significantly probably wants to tune that value upwards. You don't want it too high for low incoming datarates, but at higher rates, plausibly values like 10000 might be reasonable.
