How do I change the size of a modal sheet

I have the following code pattern:

            List {
               ....
            }.sheet(isPresented: self.$showModal) {
                SomeView(...)
            }

How can I create a sheet that fits the contents. I've tried a number of things but they only change the frame size of the contents, not the sheet itself, so does anyone know how can I control the frame size of the sheet?

(I pasted an example image here, but it disappeared when I posted the question - bug?)