Most of the UI/UX designer must have some knowledge about photoshop grid system. I have attached some PSD files to kick start your mobile and web UI design.
You know a lot of the different type of devices available in the real world like Mobile, Tab, Ipad, Laptop and Desktop.
Here I’m gonna add some reference grid size for you.
Medium And Large Device
Desktop and Laptop: it’s a 1200px grid PSD click here
You can add this grid in your photoshop then kick start your great UI now 🙂