ContentView.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        ScrollView {
            MyView()
            Text("Hello")
        }
    }
}

MyView.swift

import SwiftUI

struct MyView: View {
    @State private var size: CGSize = .zero
    
    var body: some View {
        _MyView(size: $size)
            .frame(width: size.width, height: size.height)
    }
}

private struct _MyView: UIViewControllerRepresentable {
    @Binding var size: CGSize
    func makeUIViewController(context: Context) -> MyViewController {
        let viewController = MyViewController()
        viewController.size = $size
        return viewController
    }
    
    func updateUIViewController(_ uiViewController: MyViewController, context: Context) {
    }
}

private final class MyViewController: UIViewController {
    var size: Binding<CGSize>!
    
    private let subview: UIView = .init()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .red
        
        subview.translatesAutoresizingMaskIntoConstraints = false
        subview.backgroundColor = .blue
        view.addSubview(subview)
        
        NSLayoutConstraint.activate([
            subview.widthAnchor.constraint(equalToConstant: 200),
            subview.heightAnchor.constraint(equalToConstant: 200),
            subview.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            subview.topAnchor.constraint(equalTo: view.topAnchor),
        ])
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        size.wrappedValue = subview.bounds.size
    }
}