Make sure they take no more than 15% of the screen, and are easy to dismiss.
Actually, Google now penalizes intrusive popups on mobile, and so it is recommended to keep them to maximum 15% of the screen, and make them easy to dismiss.
To be more precise, read Google’s original announcement about it.
So there are some cases where large popups on mobile are still ok, for instance to accept cookies or to have the user log in.
Otherwise, keep the popup size to about 15%.