This worked the best for me.

struct ContentView: View {
@State private var windowSize: CGSize = .zero // Holds the window size
@State private var isPresented: Bool = false // State for modal presentation

var body: some View {  
    ZStack {  
        Button("Present") {  
            isPresented.toggle()  
        }  
    }  
    .background(WindowSizeReader(size: $windowSize)) // Reads window size  
    .sheet(isPresented: $isPresented) {  
        ModalView()  
    }  
    .frame(width: windowSize.width, height: windowSize.height)  
}  
}

struct WindowSizeReader: NSViewRepresentable {
@Binding var size: CGSize

func makeNSView(context: Context) -> NSView {  
    let view = NSView()  
    DispatchQueue.main.async {  
        if let window = view.window {  
            self.size = window.frame.size // Get window size  
            NotificationCenter.default.addObserver(forName: NSWindow.didResizeNotification, object: window, queue: .main) { _ in  
                self.size = window.frame.size  
            }  
        }  
    }  
    return view  
}  

func updateNSView(_ nsView: NSView, context: Context) {}  
}

Window Size Reader: WindowSizeReader is an NSViewRepresentable that captures the NSWindow hosting your SwiftUI view and observes its size using NSWindow.didResizeNotification.

Dynamic Size Binding: The @State property windowSize is updated dynamically based on the app window's dimensions.

Apply Window Dimensions: The windowSize.width and windowSize.height are used to set the frame size.