If you have a small pressure ...

If you have a small pressure zone or DMA and a significant leak when compared with the consumption, you can check the consumption in the zone, then you can check the minimum nighttime flow (MNF). Subtract the MNF from the day before the leak was repaired to the day after it as repaired . That can give you an estimate of leak size.

If that isn't feasible, ask one of the experienced operators who was on site, how much the leak was. They can give you a rough estimate. At least they can tell you if it was on teh order of 5 gpm, 50 gpm or 500 gpm.

I've attached a chart that was developed for sewer overflow that can help estimating flows.

Theoretically, you can use an estimate of orifice size and pressure but that is even less accurate than what I've mentioned above.

If you want the total volume, you need to estimate the start time and time of shutdown.

If you get the impression that there is a lot of estimating involved, you're correct.