I have made interesting test:
Create DB
Create Table T1 ( FIELD1 VARCHAR(1600) )
insert 2 mil records of NULL. (Commit every 100.000 recs)
Size became 141 MB
And that is :
1 record
141 000 / 2.000.000 ~ 0.0705 KB or ~ 70,5 bytes. So, FB is using 70
bytes of data for I dont know what.
(As far as I remember 70 bytes , it would be ~ 35 chars)

Adomas