Size of Veenas differs from one another. Either the kudam is bigger or smaller than the other, or the Veena 'dandi' is lengthier or wider than the other. It's not necessary to record particulars of the size of Veena with a measuring tape.
A Veena has to be tested by using and trying out different gauges of strings. It's wrong to state that all Veenas must use same gauges for saarani and panchamam strings.
The gauge is determined by testing it's 'jeeva' or 'kaarvai' (sustenance and resounding naadham or resonance when the string is played upon). The sound should not be clipped, but sustained and continuous (like in the case of tambura).
In my experience the same gauge measures differently, going by the brands and music showrooms. It's important to try out different gauges to arrive at the point of honeyed sweetness only a Veena is capable of yielding.
ABOVE APPLICABLE TO ACOUSTIC VEENAS ONLY