It's hard to predict the size of the dump file. large fields will be compressed using LZ77 so the result could be larger than the table files. integers, dates, and timestamps, expressed in ASCII are larger than they are on disk.
Dumps don't have slack space, or indices, so the result could be smaller that file sizes of the database.
If you compress the dump it could compress by 90%
If you want certainty you really have to suck it and see. Eg pipe pg_dump output to a byte counter
pg_dump datname | wc --bytes