This question already has an answer here:
Down to what size is there experimental justification for modelling the electron as a point like particle without volume? Asked in another way, at what size scale would it be more correct to model the electron as a charged shell with unknown internal structure instead of a geometric point without volume?
[I am not questioning whether or not the model is correct - only where the cutoff between theory and proof should be drawn when evaluating alternative theories]
EDIT The proposed duplicate question does not contain any answers that would suggest a 'proven maximum size' like I am asking about.