Probably a Great Pyrenees:http://www.dogbreedinfo.com/greatpyrenees.htmBut I'm not sure what you consider "big," so maybe a Samoyed:http://www.dogbreedinfo.com/samoyed.htm
That could be a number of breeds like :-akbashhttp://www.bing.com/images/search?q=akbash&FORM=IGRE-Great Pyreneeshttp://www.bing.com/images/search?q=great+pyrenees&FORM=IGRE-Kuvaszhttp://www.bing.com/images/search?q=kuvasz&FORM=IGRE-Maremma sheepdoghttp://www.bing.com/images/search?q=maremma+sheepdog&go=&form=QBIR&qs=njust to name a few
I will ask you what he looks like in a photo by you going to other website.If it goes to this website it is a Maremmahttp://www.paws.com.au/BreedRescue/0maremma.jpgAnd if it is the next one its a samoyedhttp://gustiayumeliati.files.wordpress.com/2009/10/tintin_samoyed10.jpg