select owner,table_name,column_name,data_length
from dba_tab_columns
where owner='OWNER_NAME'
and table_name='YOUR_TABLE'
and column_name = 'YOUR_COLUMN';
There are a number of other columns in DBA_TAB_COLUMNS which also provide useful column information.
If you don't have access to DBA_TAB_COLUMNS, because you aren't a DBA, you can probably get the same info from ALL_TAB_COLUMNS, or possibly USER_TAB_COLUMNS.