The unit of data in Buffer Pool is Page. It means that the minimum amount of data that SQL Server reads from/to Buffer Pool is
one Page. Knowing that every page is 8KB, it can be said that the minimum amount of data that is read from/to Buffer Pool is 8KB.
If you don't know what Page is, read my post about Pages and Extents
There is an intersting point on the unit of data in Buffer Pool. Any change you make in data results in the relevant whole page be persisted back on disc.
Let's see an example. Imagine the query below which updates the Discontinued field of the People table in the Northwind database. The field is of type BIT, therefore only one single bit of data is being modified.
SET Discontinued = 0
WHERE ProductId = 8