I would like to know, will the database size remain more or less the same after data migrated to Oracle 10g (i.e. The database size for Oracle 10g is approx. to 80GB)?
The size will be increased because the default initran value for table in 8i is 1, whereas it's 2 in 10g. I do not know migration utility will take care of this.
Keep watch on big tables.