I was seriously confused until i had a long conversation with staff at http://www.reptilecentre.com - this was after various attempts to get a consistant view of what size mice should be and actually are. Prior to adopting my two adult corns I was told they were on adult mice, They are actually on extra large mice now.
as u can see from the mice section of link subz put up the weight range cover by the larger sizes is quiet wide, but bear in mind unless the all a uniform size the appearance will be diseving too, i dont tend to ask for a size as ask to see them and pic the ones i want, btu oviously you can only do that at a shop and not on net