Hole size of BGA stencil
This is a very complex matter and a higly professional task.
Please take care that (in normal cases), BGA pad dimensions will mainly depend from the BGA pitch.
Once the pads dimension is defined, normally the stencil aperture should always be smaller than the pad.
In case you need more solder volume, you have to increase the stencil thickness.
The complexity of the design depends either the BGA pads are soldermask-defined or not.