I am trying to use ASWebAuthenticationSession in order to comply with Apple's suggestions to do auth in-app instead of by flipping to Safari, but the default dialog that is shown looks a bit small to me, and I would like to make it a bit larger.
Is it possible to do any form of customization of this? It seems mostly not configurable except for prefersEphemeralWebBrowserSession and presentationContextProvider, but it is not clear to me the if the latter allows using some custom view with whatever size I define.
You won't be able to do that. It is meant to look consistent across apps, and a little different to normal windows. This informs the user that legitimate authentication is taking place in a secure window. Your logins will then look the same as those from Google, GitHub and others.
If your app looks similar to this, then it is good. Sometimes UX stakeholders find it a little different to what they are used to. But logins are both secure and user friendly: