I've seen most agates sized by the outside dimension, rather than the inside, which is modern practice. As far as use goes, I don't think it will make a difference. I know of one bamboo maker who uses size 8 Mildrum strippers for all his rods, no matter the line size, and Sage TXL's have size 6 H&H ceramic strippers for 00-4wt rods--all shoot line just fine. My personal experience is that big strippers are more important for clearing knots and loops of line when fast running fish take off, rather than for shooting line (running line, the part that shoots, is pretty thin stuff), so a 7ft 4wt should perform just fine with the smaller agate guide. If the size 2 snake looks too big for it, just start with a 1.