From the mid-body size you gave, I'd guess that your snake can move up a little in the size of your next feeders. I judge the proper size to feed much the same
way as Lord Sorril has already posted. The reptile shop staff obviously mis-spoke when calling them mice...& since they deal with both, it's not a big deal as long
as they know what YOU want, and as long as YOU know what you're buying. As already mentioned above, ask them for the next size up, as what sources call the
sizes can vary quite a bit. Many online sources (of frozen) go by weights for just this reason...if you have a small accurate scale, you can do that too, if the shop
also weighs theirs.