Imagine that you would like to design a parallel plate capacitor that could store 1 J of electrical energy in the electric field between its plates when connected to a 1.5 V battery.
Imagine that you can arrange things so that the plates can be separated by a distance d as small as 0.5 mm.
(Hint argue that the electric field strength between the plates is E approximately equal to V/d, where V is th potental difference between the plates).