How can I configure my iPhone to show my contacts photos full screen when they call or I call them I used to see my contacts' photos full screen when they called or when I called them now I only see a thumbnail when they calling and when I call them.
Have you synced your address book with GMail?
It usually does it.

It was found earlier that if you add photo to the contact through iPhone it will display them full screen. And if you add it from address book app from a mac it will display them small.

Now I heard syncing contacts to iCloud will do just the opposite i.e. it will force all the contacts to be displayed full screen. Though I did not try.
Try to sync to iCloud if you can and report back.