How do I change the MEM option when I get this error?

# DMCOPY (CIAO): WARNING: Creating large image: 999 MB. Current max set at 500 MB.
  Increase maximum using [opt mem=n] or increase blocking to reduce size.

Please note that this message is just to let you know that large files are being created. The files themselves are still correct and usable. CIAO 4.11 increased the limit that causes the warning to be displayed from 50 Mb to 500 Mb.

To avoid the warning, specify MEM=999 by adding [opt mem=999] to the input filename; e.g. for dmcopy:

unix% dmcopy "acis_evt2.fits[ccd_id=1][bin sky=1][opt mem=999]" ccd1_bin1.fits

Be aware that the DM tools may ask for a lot of memory when making large images, so keep your machine's capabilities in mind when using this option; see ahelp dmopt for more information.