Big files have to be imported from command line.
In order to do so, upload your sql file on the server, say in the /root folder.
Now let's assume your file is called backup.sql, in order to import it, you will run following command:
# change in the directory where the backup file is:
# import the file:
mysql -u YOUR_DATABASE_USER -p YOUR_DATABASE_NAME < backup.sql
One you run the second command, you will be asked for a password for YOUR_DATABASE_USER, so insert that and wait for the import to complete, this can even take hours.