Don't get to discuss Shin Guards very often...
 

I also have Gerry Davis by Wilson. Comfortable-only two straps on the lower leg so no chafing behind the knee. Do not ride up. I got hit with a foul ball at college level showcase this past sunday right on knee cap and did not feel a thing. I cut the ankle guards off today while cleaning them and getting packed for International Umpires Camp in Atlanta this weekend. I wear NB 450 high top plate shoes so the extra ankle padding was superflous. Price is right at around $50 bucks or so. I am 5'9" and wear the 17.5 inch size. I use GD stance and I put my hands on top of knee/thigh and rest them behind the knee cap protector. Thus, no foul balls to the knuckles/metacarpals. Works for me.

My two cents, your mileage may vary.

how do you measure?

Go here. Those are the instructions for Honigs, but the method is the same. I would only reduce the measure by 3/4" for high-top plate shoes. Because I wear Tanels, the straight measurement works best for me.