Tente
wmic process where name='firefox.exe' get WorkingSetSize
Paraarmazenarcomoumavariável,tente(use%%
seestiversendoexecutadoapartirdeumarquivobatouúnico%
seestiverexecutandoapartirdopromptdecomando)
for/f%ain('wmicprocesswhere"Name='outlook.exe'" get WorkingSetSize^|findstr [0-9]') do set "var=%a"