Hi guys simple question, If I do a full backup of 20G database will the backup file also be 20G long.
Not always. If you have a filegroup, said 10GB, and it is fill only 5GB, then backup will store only 5GB. Now, also SQL store the filegroup size, so when you try to restore, you have to have 10GB of space.
Whats the problem in performing the database backup to a file and see the size of it? Refer what Luis said above.