I want to make a big poster out of my daughters picture which I have on my iPhone?

But my photographer says he can't make it cuz the pixels will expand and won't come out good. Is there any way to enlarge it without effecting the pixels of photo?

You may try a program called Graphic Converter to re-size it but taking a snap shot sized picture to poster size is going to give you a blurred poster. Get a camera that takes mega pixel sized pictures and use those files with Graphic Converter. Even then you may not get poster sized prints.