You may be able to specify memory allocated to ASH by setting the _ash_size (in bytes) instance parameter, but experience has shown Oracle does not always respect this setting.

By default, ASH is enabled on all Oracle Database 10g and above systems. With the instance parameter statistics_level=typical (default), ASH will be enabled. You can also directly turn ASH on or off by setting the instance parameter _ash_enable to either true or false. By default, when ASH buffers are written to the AWR tables, only one out of every ten samples is written. This is controlled by the _ash_disk_filter_ratio parameter, which has a default value of 10.

Personally, I would never change ASH parameters unless I had a very good reason for doing so. Even more risky would be to disable ASH. As Oracle turns increasingly to automated performance management, it needs the best performance data available, and it gets much of this data from ASH.