Well I fixed the problem. For future reference for anyone else who might experience similar problems and find this thread, the trick is to set the width and margin-left to auto, like this:
(In extra.css or directly in the message user info).
thread_view .messageList .messageUserInfo a.avatar...
Hello
I have followed the instructions here to create a nicer avatar size (300 x 150), which looks fantastic on the pc, but it looks pretty bad on mobiles and squashes all the post content to one side. Is there a way to have the avy size shown on message_user_info template depend on platform?