I put a gist together where you can see an example that may work for you. The App starts with a ContentView, which launches the WelcomeView based on a button press. The gist is here.

The project is a newly-created SwiftUI project targeting only macOS, I only included the main Swift files. I experimented with getting the main ContentView created by the project to be constrained to a specific size, and found that this approach doesn't work in that case. If this solution doesn't help you, you may want to post a minimal (but complete) setup where the problem exists.