Which version of PIX do you have ?
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/cmdref/df.htm#wp1067379
If the DNS fixup is disabled, the A-record is not NATed and the DNS ID is not matched in requests and responses. By disabling the DNS fixup, the maximum length check on UDP DNS packets can be bypassed and packets greater than the maximum length configured will be permitted.
The PIX Firewall drops DNS packets sent to UDP port 53 that are larger than the configured maximum length. The default value is 512 bytes. A syslog message will be generated when a DNS packet is dropped