Sound takes place in three dimensions. I know of no measuring device that can tell me if I captured the scale of the instruments correctly. My ears will though and that’s my point.
Well whats your vantage point?? Are you 10th row of way in the back?? That's all going to change your sense of scale no? I always was under the impresion that for the most part the sense of scale was manufactured during the whole production and mastering process. Are you talking a pair of microphones going direct to disk/tape?? In that case you really wouldn't know until after the fact but you could certainly tape a rehersal and go from there.
Rob