gdt
It never occurred to me that there wasn't tagging for a vernal pool, but indeed there is not.   It'd definitely a subclass of wetland, so I just did that and added wetland=vernal_pool, which is the obvious choice.

As for size, I was standing there and would have said 10-15m across.  But, the new LIDAR is much better resolution, and I drew it from a combination of LIDAR and memory.