Whilst there is a built-in format ( SizeKMG) this will not work in the Terabyte range. Thankfully ChrisNZ provides us with a 'roll your own' format:

%let onetb=%sysevalf(1024**4);
proc format ;
  picture sizekmgt 0       -< &onetb. = [sizekmg10.1]
                   &onetb. -  high    = '000000.9TB' (multiplier=%sysevalf(1/&onetb.)) ;
run;

Usage:

data demo;
  format size sizekmgt18.;
  size=123; output;
  size=12345; output;
  size=123456789; output;
  size=123456789012; output;
  size=123456789012345; output;
run;

Which gives: