Chat app Iustration, pure css and html

If someone could help me with the phone size I would appreciate it. I kinda put a fixed size but maybe is a way to do it more responsive!

Also I'm glad to say that it's the project with which I reach 150 points that was a personal goal and also opens my Talent Profile.

Finally I'll say that if u also want to give me any help or correction of anything there are most welcome and I'll appreciate your time invested in correcting me.

Thanks