MAX_DUMP_FILE_SIZE

Limit the size of alert log files with this parameter.

Parameter can be changed at system or session level.
i.e. - alter system or alter session

Options are UNLIMITED or a number with size attributes, i.e.: - K, M, G.

To make life easier set this parameter to UNLIMITED.

http://download.oracle.com/docs/cd/B28359_01/server.111/b28320/initparams129.htm