cambo is correct when he says that they require a higher capacity battery.
a standard 600 mah battery will power your servos but not for long. you see mah stands for mili amp hour so a 600 mah battery can produce 1 mah for 600 hrs or 600 mah for 1 hr.
some will say go to 6 volt battery. all that does is make your servos faster. to answer your question perhaps a 1200 or 1500 mah battery will do you. always check with an expanded scale volt meter, to check the true charge of your battery under load when in doubt