I’m fairly new to this but would like to share…
So far, personally, I pick average size for cell phone (based on rating / statistic). The source I went off of tells me that’s 360 x 640 (that’s something like 11% of phones - but represents the largest segment). I design one design for phones (one layout for landscape orientation and one for portrait). I don’t try to represent every screen size for mobile phones, just the one representative one. I know (or could communicate) that the sizes and spaces between will be reponsive for larger / smaller screens (though 360 x 640 is pretty small - like wal mart $35 cheapie small). I follow the same principle for tablet (I think the largest segment of tablets is like 768 x 1024 right now). Ultimately, I end up with 4 breakpoints and 6 layouts. Breakpoints: mobile phone, tablet, laptop (or desktop), and large screen (eg: desktops with really big screens like 19 inches and above.
Mobile Screen Resolution Stats Worldwide