Unless you are particularly constrained but what will fit on your lathe or available material then a lot will dome down to what looks right as well as what functions.
Two flywheels here, the one on the right is the 100mm one as Stew's design and the one on the right reduced down to 80mm dia. To get a similar moment of inertia the rim has to be beefed up to 25 x 12, the spoke web thickened from 6mm to 8mm. and the hub increased from 16x20 to 20x25.
View attachment 147148
Both will work the same but I'd find the larger one much nicer to look at on the Horizontal mill engine, if it were a vertical then the smaller one would not look out of place.