The PIX 515 and 520 are limited to a config size of about 1 MB. The 525 and 535 support a config size of 2 MB after 5.3(2) code. However, if you are using PDM to manage your PIX, we do not recommend a config larger than 100MB as you will have terrible performance in PDM with a config bigger than this. You can use the 'show flashfs' command to see how big your config is: