Blog page, if statement to make the first post different

Hi,

I am looking at making my blog feed have a different first post to the rest in the page, so that the first one is fullwidth with a different sized featured image and the others are half width. I presume I need to use an if statement, but I was wondering is there a way to do it so wordpress would understand it was only to use different code for the first post in the feed?

Thanks,