Generally speaking, the larger the muscle group which is the target of the stimulation, the larger the current that will be needed to achieve the intended result. Similarly, the further apart the electrodes are placed, the greater the required current to achieve outcome. If a 'large' current is put through smaller electrodes, the current density (mA/cm2) will increase and as a general rule of thumb, higher current density results in greater discomfort (and if pushed far enough, can lead to skin irritation and eventually a burn)
You can get a result with smaller electrodes, but patient (whether human or animal) comfort will be improved with larger electrode sizes, and therefore compliance will increase
I am sure that someone, somewhere has a 'formula' but in humans - it is easy to imagine - for stimulation of the quads, it can be achieved with 5 x 5 cm electrodes, but the same result with less discomfort will be achieved with say 10 x 5 electrodes. I might use 5 x 5 on forearm or maybe deltoid or biceps, but personally would not be using 5 x 5 on the quads or hamstrings or even the gastrocs/calf
If in doubt, I would tend to go larger
Would be interested to hear what others think?