#how do i know the specific measurement for mobile device

#when next am building any project, which device size should i focus on. e.g desktop or mobile

would really appreciate it alot if someone in the community could help me in code refactoring because i understand the term DRY in programming which means do not repate yourself. i have a lot of width and height measurement in code and i can't seems to find a way to stop measuring.

#Am looking forward to become a fullstack devloper, what are the best pratice and step should i follow.