I just ordered the 10mm inzer forever single prong today. My waist size around my belly button fluctuates between 31-34 inches when measured with a soft tape. I ended up choosing a medium (33-35) since I figured it's more likely that the longer I lift, the bigger my waist will get. Also I read around on other forums and it seems that there is some leeway in belt size, so if you are a few inches smaller or larger, it should still fit fine.
As for the myotape, I once had a knockoff cheap-o version of those and whenever I measured with it, I usually ended up 1 or 2 inches shorter than if I had measured with a normal soft tape. I think the reason for that is they count the "plastic holder thing" as 1 or 2 inches and if it isn't flat against your stomach then the reading might be off. Dunno the definite answer to that though, it's just a guess on my part. GL