There has to be search function for this forum.
I am sure this has been asked many times.
How do I change the screen size?
Personally, I don't like the Stack Exchange search. I use google instead, with
site:askubuntu.com to search here specifically, like
change screen size site:askubuntu.com.
Though often there's helpful info on other sites like Super User, Unix & Linux, How-to Geek, Ubuntu Forums, etc, so I just put
ubuntu in the query, like
ubuntu change screen size.