Voltage for a electrolytic cell:
EMF definition: 1 Volt = 1 Joule/Coulomb.
Joule= Required amount of energy to force the electrons to move between two electrodes (oxidation at one electrode and reduction at the other). This force is depending on concentration of available ions.
Concentration of available Ions is depending on number of moveable electrons/mole for the chemical structure.
When there is a difference in concentration of ions between two electrodes can we measure this as a voltage.
Actual cell voltage is depending on what chemical components that are involved. When you know these and their number of moveable electrons/mole can you calculate unloaded cell voltage.
Some examples
Scaling cell size is a energy parameter and not a voltage related parameter.
Battery is a current source. If there is no stream of electrons (current) available when battery is loaded by a resistive load=>Useless battery.
This is a very simplified description as voltage for an electrolytic chemical cell also is very depending temperature and other physical circumstances.
For a deeper explanation, try in a chemical forum.