SGA resizing history

SELECT component, oper_type, oper_mode, parameter, 
initial_size/1024/1024 init_MB, 
target_size/1024/1024 target_MB, 
final_size/1024/1024 final_MB,
status, start_time, end_time 
FROM V$SGA_RESIZE_OPS;

if you want to monitor how much memory is available for resizing use the query:
select * from v$sga_dynamic_free_memory;