How to define the size (in fragment) of elements in a VStack?

Question

I'm a newbie in SwiftUI and I'm use Playgrounds tuto to learn SwiftUI.

In this example, I'm looking how to have the first circle taking the half screen and the two others circles the other half screen (sizes : 1/2, 1/4, 1/4).

import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack {
            Circle()
                .foregroundColor(Color.blue)
            Circle()
                .foregroundColor(Color.red)
            Circle()
                .foregroundColor(Color.green)
                
            }
        }
        
    }

I don't want to define the size in pixels, but relative to the screen as we do with HTML/CSS.

Thank you for your answer.

Jerome

I have seen there is LazyDataGrid but I'm not sure it's the solution.