ContentView.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        SquareLayout {
            Button {
                
            } label: {
                Color.green
                    .frame(width: 320, height: 640)
            }
        }.clipped()
    }
}

struct SquareLayout: Layout {
    func sizeThatFits(
        proposal: ProposedViewSize,
        subviews: Subviews,
        cache: inout ()
    ) -> CGSize {
        CGSize(
            width: 100,
            height: 100
        )
    }
    
    func placeSubviews(
        in bounds: CGRect,
        proposal: ProposedViewSize,
        subviews: Subviews,
        cache: inout ()
    ) {
        let size = ProposedViewSize(
            width: 100,
            height: 100
        )
        subviews[0].place(
            at: .zero,
            proposal: size
        )
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}