How to make font size customizable in a macOS app

Here is an approach with a view modifier .dynamicFont that takes a factor.

Be aware that the standard sizes (for factor 1.0) reflect the standard macOS styles. You might cover iOS sizes as well there if you want to use it cross platform.

struct ContentView: View {
    var body: some View {

        HStack {
            GroupBox {
                VStack(alignment: .leading) {
                    
                    Text("Hello, World!")
                        .font(.title)
                    
                    Text("Hello, World!")
                        .font(.body)
                    
                    Text("Hello, World!")
                        .font(.caption)
                }
            }
            
            // example use
            GroupBox {
                VStack(alignment: .leading) {
                    
                    Text("Hello, World!")
                        .dynamicFont(.title, factor: 1.2)
                    
                    Text("Hello, World!")
                        .dynamicFont(.body, factor: 1.2)
                    
                    Text("Hello, World!")
                        .dynamicFont(.caption, factor: 1.2)
                }
            }
        }
    }
}




enum DynamicFontStyle {
    case largeTitle
    case title
    case headline
    case body
    case caption
    // ...
    
    // the standard sizes and weights are for MacOS!! and taken from
    // https://developer.apple.com/design/human-interface-guidelines/foundations/typography/#specifications
    
    func specs(factor: Double) -> Font {
        switch self {
        case .largeTitle:
            return Font.system(size: 26 * factor, weight: .regular)
        case .title:
            return Font.system(size: 22 * factor, weight: .regular)
        case .headline:
            return Font.system(size: 13 * factor, weight: .bold)
        case .body:
            return Font.system(size: 13 * factor, weight: .regular)
        case .caption:
            return Font.system(size: 10  * factor, weight: .regular)
        }
        // ...
    }
}


// View expension for ease of use
extension View {
    func dynamicFont(_ style: DynamicFontStyle = .body, factor: Double = 1) -> some View {
        self
            .modifier(DynamicFontModifier(style: style, factor: factor))
    }
}


// View Modifier
struct DynamicFontModifier: ViewModifier  {
    
    let style: DynamicFontStyle
    let factor: Double
    
    func body(content: Content) -> some View {
        content
            .font(style.specs(factor: factor))
    }
}