The most common SR is a 5/8 size but there is some other considerations. Some suggest that a 6/8 is the beginners razor but in my experience a medium hollow ground grind is a thing to consider above sizes. If you say that the upper lip is a trouble zone then you must stick to that 5/8 or even consider a 4/8 razor. I mention the grind because, the stiffer the blade, the more attention to the shaving process.