A sportwatch is good around 40 / 42 mm.
A toolwatch can be even bigger.
A Grande Complicaton needs some place, so I would say somewhere between 42 and 44 mm.
A dressy watch, time only, cannot be, in my eyes, big, it is antinomic.Less than 40 is much appreciated. 35 to 38 is perfect, for me.
So, in my opinion once again, there is no ideal size, it all depends on the watch, the volumes of the watch ( sme are lacking coherence ), the category, the use, and also the culture of the owner, his wrist ( a dressy 35 mm watch on a 25 centimeters wrist may look weird ) and so on...
Some brands go higher, some don't.
And the small vintage watches have more and more success...
Best,
Nicolas