I think which size you get will depend on how wide your ering's band is, what ring size you wear, if you want to wear it by itself without the ering and want something bigger, and how long your fingers are. I just took a ruler to my finger and said "I like this stone size." But you might want slightly bigger depending on those variables. I know lots of ladies here buy cheap CZ rings in steel/silver or try rings on in stores before deciding which size is right for them.
Really like the Euro shank ring, too! Didn't know they made them in Euro. I assume it's not a full eternity, though. Does Euro shank really fix the spinning problem? That sounds like a good alternative for those who want to be able to resize their rings more easily (assuming it's not full eternity).