1. It doesn't mean that it has used the max extents. It means that total datafile size for that tablespace is 1000M and out of that 997M has been used.

2. You should see the object segments(tables, indexes) in that tablespace by querying DBA_SEGMENTS.

3. Resizing is better if have the disk resources else you may have to add datafile in another disk.

Sanjay