To look like real shutters, they should fit in the window opening trim if they were closed since their original purpose was to protect the opening from bad weather and/or allow privacy while maintaining ventilation. The inner edge of the shutter should be at about the middle of the jamb trim.

The installed shutters will touch the siding and the raised casing trim so they will be slightly askew unless the outer edge is shimmed out from the siding. The best way to do that is to use real shutter hardware especially if you're going to make the shutters.

https://www.lifeofanarchitect.com/residential-architecture-101-shutters/

Shutters are not exterior wall decoration located near a window. Shutters should be hinged, and cover the window for whatever the reason, and swing out of the way to each side of the window when they are not needed. That determines the size.

So basically unless he builds fully functioning shutters there shouldn’t be any shutters. Hmm, not sure he’s gonna want to do that.

Why do you think faux shutters are needed? Maybe we can help with ideas if you think your home’s exterior needs something more?

Ok, I understand. That makes sense.

But ideally they would function.

Why do you think you need shutters?

We installed shutters that are almost 1/3 the width of the windows, and almost the same length of the windows. I don’t think there are any rules; you just have to choose what looks appealing to you.

To look functional, they would roughly be sized and positioned like this:

Overlapping the side trim on each side with the width being 1/2 that of the Sash area.

Length would not include the "sill". Your windows do not have a sill since they are picture-framed in trim so you would make them a little shorter to indicate the sill area.

And if you are making louver shutters the louvers would face UP in this position, so that they faced down when they actually covered the windows.

If you are doing batten shutters, no spacing between the battens.

If you have some areas with problems of width, you can cheat them a little bit to the narrow side and they will still look functional.

Shutters became decorative only after air conditioning became commonly available. They were used every night in my grandmother's house.

The shutters should be the size of the space between the side and top "window casings" (see diagram) and the top of the sill whether they are operabe or not.

For an inoperable shutter, align the inner edge of the shutter panels approximately on the centers of the side casings. To make it easier to secute the outer edges, put small spacers behind the outer corners so the panel is parallel to face of the house.

Thank you all. This info is very helpful.

"I don’t think there are any rules; you just have to choose what looks appealing to you."

Uh, no. Just no.