Initialization Parameters

BACKGROUND_DUMP_DEST->describes the location where background process trace files are.

USER_DUMP_DEST->location of user session trace files are/(will be) located

MAX_DUMP_FILE_SIZE->maximum size for trace file.The default value for this parameter is UNLIMITED which means no restrictions on trace file size.This parameter applies for all types of trace files.If a value is specified for this parameter, it must be a numeric value, optionally suffixed with the letter K (kilobytes) or the letter M ( megabytes). If no suffix is provided means number of blocks.

*NOTE
Some parameters are not modifiable by alter session set command.To determine this you should query the view $parameter and see the column named "ISSYS_MODIFIABLE" appropriate values are false or true.If the value is false you should open database:
SQL>startup open;
SQL>create pfile from spfile;
edit data in newly created pfile save it and shutdown database:
SQL>shutdown immediate;
start database by non default parameter file:
SQL>startup pfile=(path of newly created pfile) ;
SQL>create spfile from pfile;
SQL>shutdown immediate;
startup normally:
SQL>startup;

Or  Simply,if it works:

SQL>alter system set parameter_name=value/'value' scope=spfile;

SQL>shutdown immediate;

SQL>startup;