Change appearance latest post

Hi guys!!

I want to try to change the appearance of just latest post in the single.php. To be honest, I dont know how to start.
My currently layout shows all pictures’ size at same, well, I would like to widen imagen’s size of latest post and make some other changes.
I guess changes must be made in single.php.

how can I differentiate betewwen latest and others??.
Some guidelines?

https://www.igmosi.com/blog