I agree with Art's definition of a microshell as being more a matter of
detail than absolute size.  There are many spp in the 4-7mm category
that need no magnification (with normal eyesight and/or good eyeglasses,
ie.) to identify, but others which are very detailed (such as Turrids
and Triphoridae) in sculpture , may require a loop or scope to examine
clearly even if they are up to 10mm.  So, a 10mm shell with much fine
detail may be a "microshell", while a 4mm specimen which is easily
examined without aids may simply be a small shell.  As for "miniatures",
this is a highly sujective term.  I do not consider a 10 mm Trochid to
be a "miniature" specimen, but others might!
 
                                                                                -Ross M.