解决了这个问题，所以我会在以后帮助任何人解决这个问题。设置你的.maximumSize为你的首选大小。然后在设置窗口后，使用DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {再次设置.maximumSize，但这次你想要的是实际的最大窗口大小。
我的完整代码：

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let windowScene = (scene as? UIWindowScene) else { return }
        
        #if targetEnvironment(macCatalyst)
        let toolbarDelegate = NewSchoolworkToolbar()
        let toolbar = NSToolbar(identifier: "main")
        windowScene.title = "New Schoolwork"
        if let titlebar = windowScene.titlebar {
            titlebar.toolbar = toolbar
            titlebar.toolbarStyle = .unified
            titlebar.separatorStyle = .shadow
        }
        #endif
        
        if let windowScene = scene as? UIWindowScene {
            let window = UIWindow(windowScene: windowScene)
            let newClassworkView = NewClassworkTableViewController()
            
            
            windowScene.sizeRestrictions?.minimumSize = CGSize(width: 400, height: 500)
            // This will be your "preferred size"
            windowScene.sizeRestrictions?.maximumSize = CGSize(width: 500, height: 800)
            
            window.rootViewController = newClassworkView
            self.window = window
            window.makeKeyAndVisible()
        }
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
            // This will be your actual size.
            windowScene.sizeRestrictions?.maximumSize = CGSize(width: 9000, height: 9000)
        }
    }

按照迪伦的建议

func tidyCatalystWindow() {
    #if targetEnvironment(macCatalyst)
    UIApplication.shared.connectedScenes.compactMap { $0 as? UIWindowScene }
     .forEach { ws in
        ws.sizeRestrictions?.minimumSize = CGSize(width: 500, height:800)
        ws.sizeRestrictions?.maximumSize = CGSize(width: 500, height: 800)
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
            ws.sizeRestrictions?.maximumSize = CGSize(width: 9000, height: 9000)
        }
    }
    #endif
}

就是这么简单。