I came here to determine how much storage a boolean variable uses

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/datatype.html
Description:

I'd like to see a fourth column added to Table 8-1 that contains the size or
size range of the data type. Maybe a note about the amount of space in the
row that a NULL column uses vs a NOT NULL.
Troy.
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