Depth maps quality:
Ultra High - full photo resolution is used, 14204x10652
High - 1/2 of original res. - 7102x5326
Medium - 1/4 of orig. res. - 3551x2663
...
...
From you photos you can estimate(or quite preciselly calculate) how big area covers one pixel in reality...e.g. if 1pix is 3x3cm, your maximum precision is 3cm. If 10cm precision would be enough for you, then you can set High(6cm precision) or Medium(12cm precision) quality.
Build mesh:
Face count - you can't really tell if high/med/low or custom value will be enough until you see the final result. So it is trial/error task. City area would need more polys, and flat areas less....every project is different.
Try create mesh on some smaller project and you will be able guess better if e.g. 500k polygons is enought for some object/area. Then try to guess what polygon count will be enough for your big project. The bad is, that metashape is still creating much denser model, which is in the last step decimated to your desired polycount.
I am often using just point cloud for presentation(
https://github.com/potree/potree ), because size of points can be adjusted and I still have option to measure everything.