GeometryReader in SwiftUI will give you the size at is available to layout the view

GeometryTestView.swift

import SwiftUI

struct GeometryTestView: View {
    var body: some View {
        GeometryReader { geometry in
            VStack {
                Text("view width: \(geometry.size.width)")
                Text("view height: \(geometry.size.height)")
                    HStack(spacing: 0) {
                        Rectangle()
                            .fill(Color.red)
                            .frame(width: geometry.size.width / 2)
                        Rectangle()
                            .fill(Color.green)
                            .frame(width: geometry.size.width / 2)
                }
            }
        }
    }
}

struct GeometryTestView_Previews: PreviewProvider {
    static var previews: some View {
        GeometryTestView()
        .previewLayout(.fixed(width: 600, height: 300))
    }
}