Swift, entire iOS app, 75 bytes

project.pbxproj not included.

import SwiftUI
@main struct A:App{let body=WindowGroup{Circle().padding()}}

Preview on iPhone Xʀ:

1

Ungolfed

import SwiftUI
@main
struct MyApp: App {
 var body: some Scene {
 WindowGroup {
 ContentView()
 }
 }
}
struct ContentView: View {
 var body: some View {
 Circle()
 .padding()
 }
}

Swift, entire iOS app, 152 bytes

project.pbxproj not included.

import SwiftUI
@main struct A:App{@State var s="1"
var body:some Scene{WindowGroup{VStack{TextField("",text:$s)
Circle().frame(width:.init(Int(s)!))}}}}

The unit for the radius is 0.5pt, which is either 1px or 1.5px depending on the device. The app crashes if you type something that isn't an integer.

Preview, with a radius of 50pt:

1

Ungolfed

import SwiftUI
@main
struct MyApp: App {
 var body: some Scene {
 WindowGroup {
 ContentView()
 }
 }
}
struct ContentView: View {
 @State private var text = "1"

 var body: some View {
 VStack {
 TextField("", text: $text)

 Circle()
 .frame(width: CGFloat(Int(text)!))
 }
 }
}