How to access safe area size in SwiftUI?

You can use a GeometryReader to access the safe area.
See: https://developer.apple.com/documentation/swiftui/geometryreader.

struct ContentView : View {
    
    var body: some View {
        GeometryReader { geometry in
            VStack {
                Spacer()
                Color.red
                    .frame(
                        width: geometry.size.width,
                        height: geometry.safeAreaInsets.top,
                        alignment: .center
                )
                    .aspectRatio(contentMode: ContentMode.fit)
            }
        }
        .edgesIgnoringSafeArea(.bottom)
    }
}

But FYI: The safe area is not a size. It is an EdgeInsets.