[QUOTE][i]Originally posted by mary [/i]
[B]OK. I added a datafile using command:
'alter tablespace /tablespace name/ add datafile /datafile name/ size 50m;
For some reason through GUI tool it says my datafile size is unlimited. ??? Is it right?
[/B][/QUOTE]

No, if you set it to 50M then it is 50M. It's either a bug of your GUI tool or you've overlooked something. Are you sure it is not showing unlimited for MAXSIZE, not for file size?

[QUOTE][i]Originally posted by mary [/i]
[B]
How do I look it up through sqlplus?
[/B][/QUOTE]

select bytes from dba_data_files where file_name = 'YOUR_NEW_FILE'

[QUOTE][i]Originally posted by mary [/i]
[B]Also, I would like to alter it using: alter database datafile /datafile name/ autoextend off next 50m maxsize 500m and get ORA-00900: invalid SQL statement
When I set autoextend on it works fine.
What am I missing here?
[/B][/QUOTE]

What is the point in seting the autoextent parameters (NEXT, MAXSIZE) if you switch autoextending off? Oracle obviously finds no good reason to set those two parameters if you set autoextending of at the same time.