For regressions on that kind of volume, you could try having REALMEMSIZE close to the total amount of available RAM, maybe around 14 GB depending on what else is running.
Option MEMSIZE should be larger still, since it includes the paging space.
Note that option SUMSIZE only affects procedures MEANS, OLAP, REPORT, SUMMARY, SURVEYFREQ, SURVEYLOGISTIC, SURVEYMEANS, and TABULATE, so not regressions.
To see how much RAM is available for SAS to use at any point in time, you can run
%put Free RAM = %sysfunc(putn(%sysfunc(getoption(xmrlmem))/1024**2,comma10.))MB;
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