Is there a way where I can define custom screen size of 2560x1600 for desktop screen?
I've tried changing resolution in config.json, but didn't work.
You'll have to create a custom device. Go to Snippets > Device > Customize to get a nice example.
Or, if you happen to have a screen with a 2560x1600 resolution, detach the canvas and preview in full-screen. :-)