"All that said, if I can find pistons that have a 21 mm pin and the right compression height, I say let them drive the bore diameter. Else, what is on the larger but still safe side? 96 mm? I believe the Chevy 305 pistons in the original Bob Legere 2.4 build article were 3.766” diameter Chevy 305 pistons = 95.65 mm."
JMHO. If I were going this far with a build (and I have) I would not knock myself out looking for off the shelf pistons. Custom pistons are not that expensive (especially on a 4 banger).
When I built the engine in my 83 745i, I ordered custom pistons, as well as a custom camshaft (not saying you need a custom camshaft). That way you can dial in everything to optimum; pin height, compression height, CR, etc.