SwiftUI: Show a view with its ideal size, but no larger than a given maximum size. The view must be flexible, otherwise it may stretch out of the wrapping frame. Two variants: as a custom Layout (iOS 16+), or with manual measuring using GeometryReader.

NoLargerThanLayout.swift

import SwiftUI

extension View {
    func noLargerThan(_ maxSize: CGSize) -> some View {
        NoLargerThanLayout(maxSize: maxSize) {
            self
        }
    }
}

struct NoLargerThanLayout: Layout {
    var maxSize: CGSize

    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        assert(subviews.count == 1)
        let idealSize = subviews[0].sizeThatFits(.unspecified)
        return Self.targetSize(idealSize: idealSize, maxSize: maxSize)
    }

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        assert(subviews.count == 1)
        let idealSize = subviews[0].sizeThatFits(.unspecified)
        let targetSize = Self.targetSize(idealSize: idealSize, maxSize: maxSize)
        subviews[0].place(
            at: CGPoint(x: bounds.midX, y: bounds.midY),
            anchor: .center,
            proposal: ProposedViewSize(targetSize)
        )
    }

    private static func targetSize(idealSize: CGSize, maxSize: CGSize) -> CGSize {
        // Use idealSize's aspect ratio, fit into maxSize.
        let idealAspectRatio = idealSize.width / idealSize.height
        let maxAspectRatio = maxSize.width / maxSize.height
        if idealAspectRatio >= maxAspectRatio {
            let width = min(idealSize.width, maxSize.width)
            return CGSize(width: width, height: width / idealAspectRatio)
        } else {
            let height = min(idealSize.height, maxSize.height)
            return CGSize(width: height * idealAspectRatio, height: height)
        }
    }
}

struct ContentView: View {
    var body: some View {
        let maxSize = CGSize(width: 300, height: 300)
        ScrollView {
            Image("david-marcu-VfUN94cUy4o-unsplash-200px")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)

            Image("david-marcu-VfUN94cUy4o-unsplash")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)

            Image("david-marcu-VfUN94cUy4o-unsplash-rotated-200px")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)

            Image("david-marcu-VfUN94cUy4o-unsplash-rotated")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

NoLargerThanModifier.swift

import SwiftUI

extension View {
    func noLargerThan(_ maxSize: CGSize) -> some View {
        modifier(NoLargerThanModifier(maxSize: maxSize))
    }
}

struct NoLargerThanModifier: ViewModifier {
    var maxSize: CGSize
    @State private var idealSize: CGSize? = nil

    func body(content: Content) -> some View {
        content
            .frame(width: targetSize.width, height: targetSize.height)
            .background {
                // Measure size of content
                content
                    .hidden()
                    .fixedSize()
                    .overlay {
                        GeometryReader { proxy in
                            Color.clear
                                .onAppear {
                                    idealSize = proxy.size
                                }
                                .onChange(of: proxy.size) { _ in
                                    idealSize = proxy.size
                                }
                        }
                    }
            }
    }

    private var targetSize: CGSize {
        guard let idealSize else {
            return maxSize
        }
        // Use idealSize's aspect ratio, fit into maxSize.
        let idealAspectRatio = idealSize.width / idealSize.height
        let maxAspectRatio = maxSize.width / maxSize.height
        if idealAspectRatio >= maxAspectRatio {
            let width = min(idealSize.width, maxSize.width)
            return CGSize(width: width, height: width / idealAspectRatio)
        } else {
            let height = min(idealSize.height, maxSize.height)
            return CGSize(width: height * idealAspectRatio, height: height)
        }
    }
}

struct ContentView: View {
    var body: some View {
        let maxSize = CGSize(width: 300, height: 300)
        ScrollView {
            Image("david-marcu-VfUN94cUy4o-unsplash-200px")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)

            Image("david-marcu-VfUN94cUy4o-unsplash")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)

            Image("david-marcu-VfUN94cUy4o-unsplash-rotated-200px")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)

            Image("david-marcu-VfUN94cUy4o-unsplash-rotated")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .noLargerThan(maxSize)
                .border(.red)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}