Pings are limited to 992 bytes on the outside interface of the PIX or Adaptive Security Appliance (ASA).
For more details, refer to Signature ID's and system message numbers.
For a workaround,
Ensure that signature 2151 is disabled in the configuration. The signature can be disabled by issuing the ip audit signature 2151 disable command.
The command is issued to ensure that large-sized pings are allowed through.